Morphology part 2. (#860)

* Add new "embedded morphology" `em_morphology`, constructed from a `morphology`, that: caches derived information such as terminal points, locations of sample points, branch lengths; provides services for canonicalizing locations and interpolation; and is used for the realisation of regions and locsets (see below).
* Add simple DSL for describing locsets and regions, which are abstract descriptions of sets of locations and regions on morphologies, together with a set of operations.
* Add a new class `label_dict` that associates names with regions and locsets.
* Extend `cable_cell` to hold a `label_dict` argument, which is used by a new method `paint` for associating mechanisms with regions.

Fixes #845.