Speed up plotting in seaborn.relplot by disabling calculation of confidence intervals

Update the documentation to mention additional packages used in some python examples.

Addresses some of the warnings from static analysis tools in #1084 

* fix some instances where exceptions were constructed, but not thrown
* fix use-after-move in a unit test.

* Adapt to moved readthedocs account

* Various DOC changes/updates, most importantly to:
* Single Cell Model
* Recipe

* Put generated mechanisms for each built-in catalogue in their own namespace.

* Remove blank line from `ball_and_stick.swc` 
* Mention that blank lines in SWC files indicate end of data.
* Check for trailing data after parsing in the python interface.
* Leave stream in good state if `parse_swc` encounters an empty line indicating end of data.

* Rename `as_segment_tree` to `load_swc_arbor`.
* Remove `swc_mode`: `sort_and_validate_swc` now performs the `relaxed` checks by default, `load_swc_arbor` performs the `strict` checks. 
* Change `load_swc_xxx` to only accept `swc_data` as an argument
* Remove default constructor for `swc_data` and enforce validation of SWC records via `sort_and_validate_swc` at construction.
* Add fixes for docs and unit tests.

* Add interface for querying names.

* Add derived mechanisms to list, clean-up.

* Add unit tests.

* Add python bindings and docs.

* Swap arguments in target count check in `fvm_lowered_cell_impl.hpp`.
* Update unit test to suit.

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* Fix out-of-date documentation.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.