Add optional spherical root for NeuroML. (#1464)

* Adds a `neuroml_options` namespace with enum for use with `neuroml::morphology(...)` and `neuroml::cell_morphology(...)`.
* Add support for a 'spherical' root segment with option `neuroml_options;:allow_spherical_root`; a zero-length root segment with identical proximal and distal radius will then be converted into an area-equivalent cylinder in the resultant morphology.
* Amend `place_pwlin::all_at` so that locations on a zero-length segment with different radii at the endpoint will give an mpoint for either end.
* Add `allow_spherical_root` boolean optional parameter to Python NeuroML morphology methods.
* Update docs; C++ unit tests.