- make fmt a dependency of libmodcc, not modcc
- set FMT_HEADER_ONLY as a compiler flag to avoid double definition

* Create a script that generates `diff.log` if git submodules have tags more recent that the commit they're currently at.
* A Github Action runs this script monthly.
* _If_ updates found, Github Action creates an Issue like so: https://github.com/brenthuisman/arbor/issues/9
* Actually updating the git submodule and Spack file are still manual

First step in addressing #1731

Check for errors in the solvers before the `finalize` stage and return early if errors were encountered. 

Fixes #1722.

* Add documentation for collaboration on a PR using just git
* Add the stage of Issue creation to PR workflow.
* Correct header level in test docs

Add the lmorpho utility back to Arbor

The original utility was removed because it was a maintenance burden with no users.
Now users are requesting the feature, this PR adds the old code, and updates it
to work with the new morphology description API.

* add a test that tests that a catalogue built separately can be loaded via the Python interface
* further simplification of dynamic library support
  * move all platform-specific code into the cpp implementation and out of header.

Fix various non-standards compliant code when compiled with elevated warning levels (-Wall -Wpedantic). Here is a list of the problems addressed:

**preprocessor**
- expansion of empty `__VA_ARGS__` in `ARB_PP_FOREACH`
- named variadic macro arguments in `TRACE` macro

**glibc** 
- deprecated `mallinfo` call for newer glibc versions

**simd**
- non-const C-array sizes

**modcc**
- generation of C-arrays of size 0

**switch/case**
- switch-case ranges

**tuple_foreach**
- compound literals

Use `constraint_category` only when reading/writing data sources indexed by `node_index`. Every other indexed data source assumes that the underlying index belongs to the category `index_constraint::none`. 
Fixes #1734.

Fixes #1753

Extend the `pwlin_place` interface to find the location of a morphology that is closest to a 3d coordinate.
* extend `arb::pwlin_place` interface
* python wrapper
* unit tests
* documentation

Addresses a feature request by @Helveg that I don't think ever had a ticket assigned to it.

Fixes #1661 and #1108.

* Consolidate default_concurrency, default_gpu functionality into arborenv/default_env.hpp, .cpp.
* Add new arborenv function `default_allocation()` that uses environment variables ARBENV_NUM_THREADS and ARBENV_GPU_ID to create an `arb::proc_allocation`, with a fallback to `thread_concurrency()`.
* Use arborenv-specific exceptions instead of generic std::runtime_error etc.
* Update examples, docs to suit.

Fixes #988 (except for reworking all the unit tests!)

tinyopt README.md had an example which was not updated to the 1.0 API. Fixed upstream, and this is a copy.

* Add test that is sensitive to small radius loss of precision in ixa computation.
* Add test that checks for consistent behaviour when there is an (isolated) zero radius point in the morphology.
* Special case p==1 q==1 case in ratelem so that it can interpolate ixa in the presence of non-finite interpolants.
* Change naming in pw_element/pw_elements: 'element' refers to the extent+value pairs comprising a 'pw_elements' object; 'value' refers to the value associated with an element; 'extent' refers to the closed interval which is the support of the element.
* Allow values (but not extents) in a pw_elements object to be mutable, using proxies for iterator access.
* Write piecewise zip (pw_zip) in terms of a lazy pw_zip_view. Rename zip functions.
* Document pw_elements more thoroughly.
* Simplify and document embed_pwlin.cpp routines, expressing everything in terms of simple interpolate/integrate operations.
* Represent integrated inverse cross-sectional area in the embedding by multiple piecewise-rational functions over a branch, each of which contribute separately to an ixa-based integration, so that precision loss associated with small (or zero) radii can be avoided.
* Comment embed_pwlin.cpp more thoroughly.

Fixes #1526 

Add SVE implementations of `sin` and `cos` using `std::sin` and `std::cos` respectively. 
Fixes failing CI on a64fx architectures for vectorized builds.

Some refactoring of the SOLVE statement handling in `module.cpp`. 
Allow the usage of `SparseNonlinearSolverVisitor` for non-linear systems not only when in the form of a kinetic scheme. 
Check linearity of kinetic schemes and linear systems (was previously skipped).

Adds support for user-defined gap junction mechanisms. Fixes #1600.

- API Change: Wrap `mechanism_desc` in one of `density`, `synapse`, `junction` to be able to differentiate the kind of placed mechanism between point and junction mechanisms. 
- API Change: The `ggap` parameter of a `gap_junction_connection` renamed to `weight` and is not a unit-less parameter. 
- Mechanism ABI change: New `peer_index` vector which stores the indices of the peer sites of a gap-junction-connection. New mechanism kind. 
- Internal API change: `shared_state` no longer controls the gap junction current contribution, that is now handled in the mechanism callbacks.
- Modcc change: Add `JUNCTION_PROCESS` and `v_peer` keywords to nmodl, the first to identify a gap-junction mechanism, the second to access the peer voltage of a gap-junction connection. Modify modcc to be able to compile nmodl and generate correct code.
- Behavior change: gap-junctions now contribute to both the current and conductance of a CV as opposed to just the current.
- Add new "gj" mechanism, to replace the built-in constant-conductance based linear gap-junction implementation.
- Fix unit tests.
- Fix python wrapper. 
- Fix docs. 

Add `proximal-translate` and `distal-translate` locset expressions.

Fixes #1662

Fix sub-optimal performance in external catalogues.

Fixes #1712

Add methods to `label_dict`
- iterators `keys`, `values`, `items`
- `update(dict)`
- `label_dict(iter[k, v])`, `label_dict(label_dict)`
- `__contains__`

Also bump `pybind11` to latest and clean up `label_dict` implementation.

Addresses #1535 (in so far as possible)