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 

Introduces the new policy `round_robin_halt`. This enables to halt at the current item until the policy `round_robin` is called again.

Related to issue #1749.

Fix a number of small issues regarding performance and correctness found by PVS Studio in the quest for a better code base.

# Changes
- Bump ABI to v0.1.1
- Add `arb_mechanism` to bundle `type` and `interface`s
- Adjust `generate_catalogue` and `build_catalogue`
- Return `nullptr` for GPU interfaces if not enabled

# Open Questions
- How to tell CMake not to link against `libarbor.a` and only use headers?
- ~~How to get around `#ifdef ARB_GPU_ENABLED`~~ Update: just embrace it.

# Linked Issues
Fixes #1834 

Add tutorial on how to integrate a specific Allen Institute cell DB into Arbor. 
Use the example shown at Bernstein'20.

Add tutorial on using all logical cores on a machine, facilitated by the new flag to context creation.

Extend the probe demo example with point probes sampling the conductance of two synapses.

Co-authored-by: Brent Huisman <brenthuisman@users.noreply.github.com>

As per #1731, this PR adds a dependencies version policy to the docs. It also includes a file containing a list of all configure-time deps.

Obsoletes wiki page.

Add a first round of developer documentation, evolving from the older `internal` part of the docs.

Remove checks for move ctor when it could be elided

Add accessors for the following items in `global_properties`
- ion_species (name -> valence) RO
- ion_data (name -> initial values) RO
- ion_rev_pot (name -> rev pot mech) RO
- Vm, rL, TK, Cm: RW

Fixes: #1802 

Upstream bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1006615



Currently affects Ubuntu 22.04 users.

Co-authored-by: Simon Frasch <simon.frasch@cscs.ch>

EOM

Ensure all indices into ABI variables are generated in dependency order, eg
```
idx = indices[iy]
foo = foos[ix]
```

Fix a crash in GPU code when threshold detector is used for the first time and its internal state is allocated with a wrong size.

- Add a callback to be executed between epochs.
- Add example: print progress indicator

TL;DR: Simple change for ~2% gain on GPU in a typical workload.

* add support for parsing markers defined in asc files
* add hitherto unknown "High" leaf branch type

Since we have support for ACC loading, the errors thrown by `paint` are no longer
sufficiently informative. Even before that it was already hard to correlate source 
and locus of painting errors.

Example
```
$> python3 main.C51A.py
Traceback (most recent call last):
  File "main.C51A.py", line 33, in <module>
    cell = A.cable_cell(morph, labels, decor)
RuntimeError: cable_cell: cable (cable 6 0 1) overpaints
```
This adds more details to the error message
```
$> python3 main.C51A.py
Traceback (most recent call last):
  File "main.C51A.py", line 33, in <module>
    cell = A.cable_cell(morph, labels, decor)
RuntimeError: cable_cell: Setting property 'ion-reversal-potential' on region '(region "soma_group")' overpaints at '(cable 6 0 1)'
```