1. All s-exp parsing has migrated to `arborio`
2. CV policies can now be read from string as per #1334. This is needed for the GUI project.
2. No longer can `arb::locset` and `arb::region` be constructed from mere strings
2. We have `arborio::literals` to construct `region`s, `locset`s, and `cv_policy`s in the form of `_rg`, `_ls`, and `_cvp`.
3. The `_lab` suffix now constructs an object that converts to `named` morphologies, rather than a simple string.
4. Constructors of `region` and `locset` are now properly guarded
5. Consequently, a lot of missing inheritance from `region_tag` and `locset_tag` was retrofitted.

Closes #1334 
Closes #1419 

The `label_dict` Python wrapper caches label strings and their descriptions in order to
provide a convenient Python iterator interface and `[]` access. These were not being
properly updated or set when the label_dict was constructed from a C++ label_dict.

* add neuroML example
* no test, because GH actions arbor is built without neuroML support

* Move contents of `arbornml` to to `arborio` 
* `arborio` will be built with NeuroML2 morphology support if `ARB_CMAKE_NEUROML` is set. 
* Wrap `arborio::neuroml` and `arborio::morphology_data` for pyarb.
* Add documentation.
* add `neuroml` flag to `setup.py`

Fixes #1256
Fixes #1234