Rename/hide components to fix ODR violation in arborio. (#1444)

* Add `nml_` prefix to NeuroML-related classes in arborio.
* Separate `arborio/xmlwrap` from NeuroML exceptions.
* Put xmlwrap functionality in its own namespace.
* Have xmlwrap property access errors returned by util::expected, so that the corresponding NeuroML exceptions are strictly the
responsibility of the NeuroML parsing code.
* Rename NeuroML implementation and support sources and headers.
* Place implementation classes for Neurolucida ASC parsing in an anonymous namespace.

Fixes #1441.