Memory access optimization for the fine matrix GPU solver.

Optimise the hh NMODL implementation to reduce unnecessary loads/stores and memory overheads.

Add a catalogue of mechanisms used by the BBP models.

* Add some log messages in python examples
* Add python examples to CI

* Fix some broken links and typos.
* Fix out-of-date documentation.
* Reword some of the existing sections.

Remove Python API ref page, Spack Install, fix some links

Remove imported dependencies in the documentation generation.

* Python examples: Updated to use region expressions (quotes)
* Python examples: Uniform plotting
* Python examples: Removed tutorial directory (example roughly matches python/example/single_cell_model.py)
* Python examples: Rewrote flat_cell_builder() to segment_tree()

* Synced pages between Concepts, Python API and C++ API wherever possible
    * Recipe pages conform between the three section (concepts, python, c++)
    * Cell, Cable Cell and Cell * pages are rearranged and provided with some copy explaining the relationship between them.
* Moved Python API out of Concepts
* Renamed Concepts "How does Arbor work?"
* Added Python Module Index plus mock import of Arbor for RTD build (unfortunately won't show there)
* Broke out Interconnectivity (synapses) page.
* Reworked Single Cell Model page into a quick start, with lots of cross referencing.
* Tweaked logo.
* Added Spack to install options.
* Updated blurb.
* Documentation now follows EU capitalization rules.
* Assorted typofixes

* Remove contiguous numbering requirement from 'strict' mode SWC
parsing, as it is not necessary for correct interpretation as a segment
tree.
* Describe in the swcio.hpp header how (strict) swc data is interpreted
as a segment tree.
* Demonstrate in unit tests that out-of-order and non-contiguously
numbered records are valid for `parse_swc`.
* Add unit test for `as_segment_tree`.

* Minor output formatting fix for `swc_record`.
* Modify the Python `load_swc_allen` implementation to cope with SWC
record ids not necessarily being contiguous, and with SWC record parent
ids corresponding to record ids, not 0-based indices.

* Update `dryrun` to generate some spikes again, add a voltage printer.
* Fix `ring.py` recipe.
* Fix labels in `single_cell_builder.py` and `single_cell_swc.py`.

* Simplify scanning of SWC records.
* `parse_swc` now retains metadata read from SWC comments.
* `parse_swc` is given two modes, relaxed and strict. Strict mode enforces contiguous record numbering and rejects SWC that looks like it has a single-point soma.
* Subtype `swc_error` according to error mode.
* Remove `point` class and corresponding unit tests.
* Add a RAII stream formatting flag saver, used in SWC record output operator.
* Replace included 1-point soma SWC example with a regular SWC from NeuroMorpho.

* Raise an exception if:
1. `recipe.num_souces(gid)` != number of detectors placed on the cell.
2. `recipe.num_targets(gid)`  != number of synapses placed on the cell.
3. `recipe.connections_on(gid)` has connections with non-existent source or target gids or lids
* Raise better exception for gap junctions
* Fix unit test 

Addresses #681 

* Use structured bindings where noted (there are likely other opportunities in the code).
* Use `if constexpr` to simplify some template-dispatched code.
* Remove `arb::uitl::get`, as it is no longer required now that util::variant is replaced.
* Add `_v` utility inline constexpr template values.

Part of the C++17 migration task #1022.

* Substitute `std::optional<T>` for `arb::util::optional<T>` for non-reference types T.
* Remove reference-deducing `util::value_by_key`; replace usages with new function `util::ptr_by_key`.
* Add some missing header includes that were required but included only transitively.
* Remove `operator<<` overload for optional in python/strprintf.hpp; replace with utility wrapper class that catches `std::optional<T>`.
* Wrap some `std::optional` values with `to_string` in ostream output in python lib.

Fixes #1154.

* Add CMake infrastructure for new `arbornml` library comprising Arbor's NeuroML2 (C++) support.
* Implement NeuroML2 parsing and interpretation, using libxml2 for XML parsing.
* Add associated documentation, unit tests.
* Replace `arb::util::optional` with `std::optional` in stitch morphology interface.
* Add optional prefix to `arb::label_dict` import.
* Update CI to test arbornml, with associated workarounds for OS X targets.
* Remove glob functionality from `sup`, as it is no longer needed (it was used for lmorpho) and it triggers yet another OS X issue.

Fixes #1088.

* Move label and s-expr parsing code and unit tests from Python to arbor C++ library.
* Update `locset` and `region` constructors that take string arguments to parse strings as s-expressions or "quoted" labels.
* Modify the input stream modifier used to parse asc files to use a lookup table for substitutions, renamed it transmogrifier.                                                             
* Replace `hopefully` type implemented in python headers with an `arb::util::expected`.
* Add `ARBDEV_COLOR` CMake option that forces gcc and clang to always output color output.
* Allow arbitrary strings in labels in region and locset expressions.
* Add `parse_region_expression` and `parse_locset_expression` functions alongside the existing `parse_label_expression` function for use when a region or locset is expected. These calls will promote a quoted string `"label"` to `(region "label")` or 
 `(locset "label")` respectively.
* Add user-defined string literals for labels so that the C++ interface can use `"soma"_lab` instead of awkward escaping `"\"soma\""`.
* Simplify Python wrapper code.

* Replace usages of `util::any` with `std::any`.
* Modify `util::any_cast` so that it forwards `std::any` arguments to `std::any_cast`; move into own header.
* Prevent `std::unique_any` from implicitly encapsulating `std::any` objects.
* Replace `test_any.cpp` unit tests with `test_any_cast.cpp` unit tests.
* Replace some usages of `std::is_same<X, Y>::value` with `std::is_same_v<X, Y>` as part of general C++17ification.
* Remove `util/variant.hpp`, `test_variant.cpp`.
* Replace occurrences of `util::variant` with `std::variant`.
* Replace `util::variant::get<N>` with `std::get<N>`.
* Replace `util::get_if<N>` with `std::get_if<N>`, passing a pointer to variant instead of a ref or value.
* Add some standard #includes where they were required in the source, but only included transitively.
* Some minor comment tidying and fix-ups.
* Update C++ API docs.

Fixes #1144.

Fixes #1149.

* Make the members of `cable_cell_ion_data`  optional.
* Add separate methods for painting `init_int_concentration` , `init_ext_concentration` and `init_reversal_potential` on the cell.

* Implement a workalike for the proposed `std::expected` class: see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0323r9.html .
* Replace use of `either` with `expected` in `mprovider`, `mechanism_catalogue`, `util::partition_range`, and `pyarb::hopefully`.
* Replace use of `either` with `variant` in `util::sentinel_iterator`.
* Add `in_place_t` constructor for `util::optional`.
* Fix move assignment bug in `util::variant`.
* Remove `util/either.hpp` and associated tests.

Fixes #1135. 

* Update CMake CXX version.
* Make CUDA 10 a minimum requirement and remove CUDA 9 support code.
* Set up g++ Travis tests to use g++ 8.
* Explicitly set C++14 version for nvcc.
* Properly split cuda compilation, including in unit tests.
* Remove unnecessary modcc `SOLVE` warning.
* Update pybind11 module to tag v2.5.0
* Replace `util::size` and `util::data` with `std::` equivalents.

Switch `arb::util::either` to `arb::util::variant` in the Python wrapper.

Fixes #1133 

* Fix copy-paste error that led to legitimate copies throwing a bad_variant_access exception.
* Fix exception safety for the case when an error thrown in copy assignment takes the lhs to valueless.
* Fix wrong valueless semantics associated with an exception being thrown during move assignment.
* Add unit tests that exercise these issues.

Fixes #1136.