knowledge-space

* Pyarb specific.
* `proc_allocation_shim()` throws error if user sets threads to zero.
* `arbor.context` constructor accepts `threads` set to `"avail_threads"`, which sets the number of threads to `arbenv::thread_concurrency()`
  * This introduces a dependency on arbenv for Pyarb.
* Docs and tests added.

Fixes #1692