* 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) 

* Add platform abstraction for loading DLL/SOs.
* Throw more informative exceptions when handling DLL/SOs.
* More informative errors when dealing with dynamically loaded catalogues.
* Translate `arb::file_not_found_error` to `FileNotFoundError` in Python.

- Add `simulation.clear_samplers` to remove spikes and samples from Python.
- Usecase: long-running simulations; where unlimited growth of sampling memory is an issue.

A `--debug` flag is added so that users can inspect the out-of-tree generated C++ code of their catalogue builds.

Adds the `profiler_initialize(ctx)` and `profiler_summary()` functions to the Python module and the `profiling` key to `arbor.config()`. closes #1685.

- `make install` in CI
- make `build-catalogue` play nice with Python 3.6
- execute `build-catalogue`

Fixes #1666

Limit poisson schedule with a stop time using the same semantics as the regular schedule.
closes #1617

More, subtler fallout of the ABI patchset: When mechanisms are added to empty regions, various, layered segfaults
appear.
1. `set_parameter` will receive a `nullptr` upon linear search, throwing `no such parameter`
2. Guarding this for finite `width` uncovers further need in the ABI interface methods
3. This is caused by early exit in `instantiate` on `width == 0`
4. Getting rid of this is better, as we can treat vacuous mechanism painting like all others...
5. ...but causes another segfault, since we default index arrays to `cv_pos.back()` but `cv_pos` is empty.

This patch fixes all of the above and in addition removes mechanisms without support before reifying them
on the FVM cells. Relevant tests are added.