Implements #1376.

* Provide a common C linkage ABI for externally compiled mechanisms, for both CPU and GPU.
* Remove mechanism type hierarchy (`concrete_mechanism` etc.), and move corresponding functionality to the back-end shared state objects. Mechanism catalogue is no longer indexed by type id.
* Distinguish between SIMD optimal alignment and SIMD width with new `min_align` attribute. Mechanisms provide both pieces of information via ABI.

* added notes re literal_include to python examples
* merged changes in #1504 
* deduped and includified (https://github.com/arbor-sim/arbor/pull/1558) tutorials.
* First divergence of tutorial and code spotted!
* Added some contrib documentation for examples and docs.

New structs and types:
* `cell_tag_type` (std::string): for labelling placeable items on a cell. The label refers to a number of items placed on a locset, equal to the number of locations in a locset. The number of locations in not always known to the user, so the previous way of using indices for items was no longer sufficient. 
* `lid_selection_policy`: for allowing a user to select a single item from a group of items sharing a label. Currently only `round_robin` and `assert_univalent` are supported. 
* `cell_local_label_type` and `cell_global_label_type`: for identifying the target and source of a connection or gap_junction connection. 
* `cell_label_ranges`, and `cell_labels_and_gids`: for propagating information about the labelled items on the cell from the cell groups back to the simulation and communicator. 
* `label_resolution_map` and `resolver`: for selecting an item (and retaining state) from a labelled group of items on a cell according to a user-selected policy.

Changes to the model-initialization: 
* The `communicator` now needs `label_resolution_maps` constructed from the cell group data in order to build the `connections` vectors. 
* The `simulation_state` object handles the transfer of the information when it is constructed. 
* Spike exchange at runtime remains unchanged, because `communicator::connections` remains unchanged. 

Changes to cells, cell_groups and recipe:
* `decor::place` expects a third label parameter, no longer returns an `lid_range`.
* `lif`, `source`, and `benchmark` cells need source/target labels in their constructors. 
* A `cell_group` needs to save data about the gid/labels/lid_ranges of each cell, to propagate back to the `communicator` constructor. 
* Connections/gap junction connections are formed between {label, policy} pairs on cells instead of indices. 
* `num_sources`, `num_targets`, `num_gap_junction_sites` deleted from `recipe`.

Additional changes:
* Add MPI wrapper for exchanging vectors of strings. 
* Corresponding updates to unit tests, Python wrapper, C++ and Python examples, documentation.

Fixes #1394

Fix mistakes reported by @Helveg in the Python tutorial docs. 

* Fix the regular schedule generated in `single_cell_model::run`
* Adjust frequency of probes in python examples to the correct unit (KHz). 

Fixes #1457

* Used Sphinx glossary directive to create terms that can be referred to in `concepts/morphology.rst` and `concepts/labels.rst`.
* Added note box on difference with Neuron, add source for NEURON cylinder discretisation
* Parameters added to svg generation script: turn off colors, root
* Fix cpp TOC duplication, match ordering in other sections.
* State default cv policy
* center/re -> midpoints
* Clarify and remove compartment term

Make the `cable_cell` interface read only by passing the decorations
to its constructor in a new `decor` type.

C++ library
* Remove the paint/place/set_default interface from `cable_cell`
* Create a `decor` type that:
  * stores lists of paintings and placings
  * stores a set of cable cell parameters
  * uses the `paint`, `place`, `set_default` interface that was deprecated
    from `cable_cell`.
* Create `paintable`, `placeable` and `defaultable` variants that are sum types
  over the respective types that can be painted, placed and defaulted.
* Remove the overloaded `cable_cell::paint`, `cable_cell::place`,
  `cable_cell::set_default` methods to single methods that consume the sum types.

Unit Tests
* Many small changes because many many tests use cable cell API.
* There were no `cable_cell` unit tests! Not such a big deal, since cable_cell
  is tested implicitly in so many other tests
  * but I added cable_cell tests: not much at the moment but will quickly fill
    ...

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

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

* 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()