Fixes #1236

Fix Python linking errors on some platforms.

This appears to be related to the GCC visibility of methods overridden in header files, and is fixed by moving the implementation to the corresponding cpp files. 

Addresses the Travis CI failure of #1225

* Add an example describing the content of a recipe in the recipe concepts page in the docs.
* Mention that recipes are necessary, but can be abstracted away using the python `single_cell_model` (Hasn't been documented yet).

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* Fix out-of-date documentation.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.