- Remove the old, multi-target `event_generator` class in favour of `schedule_generator(tgt, weight, explicit_schedule)`
- Much simplification ensues, `event_generator` is no longer a type-erasing container, but just what 
   `schedule_generator` was before
- Make the label resolution in generators a bit more eager, no longer at simulation time, 
   but now during setup (bit give a wee bit of perf as well)

Closes #1488 

Add functionality, docs, and examples on editing the connection table.  This is a first, small PR
on the topic, further functionality will come as requested.

Teaser example
```cxx
    // This recipe -- omitted -- implements a mutable connection table 
    // and returns it via the `connections_on` callback. New entries are
    // added using `add_connection` which makes one link from a central 
    // `spike_source_cell` to the given `gid`
    auto rec = recipe();
    rec.add_connection(1);

    auto sim = arb::simulation(rec, ctx);

    // Record spikes
    std::vector<arb::spike> spikes;
    sim.set_local_spike_callback([&spikes](const auto& s) { spikes.insert(spikes.end(), s.begin(), s.end()); });

    // Run and print all spikes per gid
    sim.run(0.25, 0.025);
    show_spikes(spikes);

    // Add a new connection
    rec.add_connection(2);
    sim.update_connections(rec);

    // Run and print all spikes per gid
    sim.run(0.5, 0.025);
    show_spikes(spikes);
```

# Update July '22

- added re-wiring of Event Generators
- implemented a split between a recipe and a recipe-update (cheekily dubbed `topping` until further notice)
- `recipes` are `toppings` via inheritance

**BREAKING** Remove access to time `t` in NMODL.

- Enhance `run_cpp_examples.sh` to do some very basic checking of spikes counts
- Add a script to check all releases
- The former must succeed for all commits to master

Here's the expected spike counts as of 0.7
- brunel 6998
- bench 972
- ring 94 (19 pre-v0.7)
- gap_junctions 30

- allow arbor to be build against py3.11
- modernise!

Document use and care of spike detectors.

Enable pyArborio input routines to consume Stream objects in addition to files by name.
- NML
- NeuroLucida ASC
- SWC

Closes #1459 

Do not abuse `pybind::arg_v("name", T default, "description")` as `pybind::arg_v("name", "description")`.
Use descriptive names and `pybind::arg` instead.

Closes #1754 

Functionality to
- delete ions from global properties
- peek into global properties
- expose painted, placed, and defaulted items
- expose voltage limit to python
- make voltage limit an optional to allow for arbitrary values.
- Add ion_settings view to python = (valence, ion_data, rev_pot_method)

Documentation
- cable cell properties in python

Closes #1578 
Closes #1577 
Closes #1607 

* Add find_private_gpu and arbor.env.

- Add stacktraces to
   1. arbor_exception (and all derived) as `where`
   2. arbor_internal_error as `where`
   3. arb_assert
- Propagate stacktraces to Python exceptions derived from the above.
- Expand dev/doc on debugging

* Clean up the bundled mechanisms. Closes #1155

Remove the redundant fvm_(value|size|index)_type aliases and replace them with the raw ABI types.

Throw error if WATCH encountered in NMODL.

- elaborate probing API
- add definitions for different kinds of probes (vector, scalar, set).
- add sketches and example for data layout

- It's actually a Github cache trick: https://github.com/actions/cache/issues/342#issuecomment-673371329
  - First run: 1h 7m 38s 
  - Second run: same duration. Hmm....
  - Third run: ~10 minutes!
  - 4th: ~7.5 min.
- Bump in configs: macos 10.15 will disappear in a month: https://github.com/actions/virtual-environments/issues/5583
  - Bump clang-max to 14, gcc-max to 11
  - macos-min to 11, macos-max to 12.

- Generate timestamped VERSION once, to properly group on Test.PyPI. Example:
  - https://github.com/arbor-sim/arbor/actions/runs/2696804929
  - https://test.pypi.org/project/arbor/0.7rc20220719101015/
- Update/simplify release documentation.
- Update to v0.7

* - Weekly CI Python wheel build pushes to Test.PyPI.org
    - Easy availability at https://test.pypi.org/project/arbor/
- Small change in documentation around VERSION
- Delete superfluous setup.py file.

Move Python build to `pyproject.toml`, bump Python minver to 3.7, fix macos wheel generation (#1916)

* Failing Macos Python wheel builds fixed.
* Macos Python wheels now come with dual-arch (x86-64 and arm64)
* Moved (nearly) all Python build instructions to `pyproject.toml`
  * Enables 'build isolation', and need to specify build-deps only once, no need for users or CI scripts to pre-install them.
  * Enables editable `pip` installs (`pip install -e ./arbor`)
  * Compatible with 'build frontends' `pip` and `build`.
  * Passing CMake options actually got shorter
* Drop Python 3.6 support.

- Fix line numbers in tutorials, fallout from #1906
- Mention arbor-contrib in a few relevant places
- Correct some fallout from #1904
- A new hardware and profiling tutorial, and covers things moved out of other examples in #1904
- Various other documentation fixes

Adds the ability to scale parameters of (density) mechanisms based on inhomogeneous properties along a cell.
Two new types are added:
- iexpr: An expression describing the scaling factor computation.
- scaled_mechanism: A wrapper struct around a mechanisms with iexpr attached to selected parameters.

Closes #1650

Post-Merge clean-up for the diffusion PR
- remove redundant `WRITE`s in mechanisms
- diffusion.py is now closer in functionality to diffusion.cpp
- diffusion.py now prints a table of results

Encapsulates 80% of cases:
```cxx
 rec = recipe()
 ctx = make_context()
 dec = partition_load_balance(rec, ctx)
 sim = simulation(rec, dec, ctx)
```
is now written as
```cxx
 rec = recipe()
 sim = simulation(rec)
```
In python we use keyword args to allow
both to be specified separatly.

Partially fixes #1862 

# Introduction

Plasticity processes are mediated by signalling ions, eg Ca++, which are generated by 
synapses upon reception of a spike. This adds a quantity `Xd` for any ion `X`
- initialised as `Xi`
- read from and written to by NMODL density and point mechanisms.
- propagates according to a diffusion law `∂_t Xd = ∂_z c X ∂_z Xd + iX/qi`.
- in contrast to `Xi` and `Xo` there's no buffering and the update in mechanisms
  occurs atomically (and at a different time)

More details can be found in the documentation.

For the future there are some low hanging optimisations
- per-ion conductivity for the matrix solver could be disabled if no diffusion is computed
- cable and diffusion solvers store duplicates of the solver state, could be merged

Closes #1651