Add flat morphology representation to `nestmc` lib. (#176)

This PR is a prelude to closer integration of the random morphology generation with the miniapp, with the first step being support for recipes that create cells from morphologies generated off-line. It aims to use nest::mc::morphology as the flat morphology-only representation that can be used to construct nest::mc::cell objects and which can exist as a target for SWC conversion and random morphology generation.

Simplify swc io implementation:
Avoid throwing exceptions in istream parsing and swc_record constructors — only throw when explicitly checking consistency, or when parsing a full sequence of records.
Allow direct access to record members.
Separate parsing considerations from canonicalization (renumbering, sorting) of a sequence of records.
Move lmorpho morphology classes into src/
Add invariant check procedure for morphology.
Make cells via swc -> morphology -> cell building, rather than direct swc -> cell.
Allow option to use ...