Add a `restrict` locset expression type that returns all locations in a locset that are in a specified region.

Also allow region and locset names with hyphens.

Fixes #1031 
Fixes #1032 

Add `maxset` function that returns the most distal set of locations in a location list, similar to existing `minset` function.

Use `minset` and `maxset` consistently in `most_proximal` and `most_distal` locset expressions respectively.

- Rename z distance region definitions to refer to the root, not the soma.
- Also fix printing of some region expressions to s-expressions.

Fixes #947.

A big update that wraps over 90% of the public C++ API, with enough functionality to let Python users to perform useful modelling.

Key features
- wrapping of cable cell functionality
  - user-defined explicit compartmentalisation not supported
- `single_cell_model` abstraction unique to the Python wrapper that simplifies developing and testing single cell models
- one-dimensional cell builder for Python wrapper that simplfies building cells that 
  - in and of itself limited in scope, but a very useful example of mapping a richer morphology builder onto `sample_tree`s.
- parsing of region and location expressions from strings
  - implements a generic s-expression parser that we can use for other purposes later