Extends the available locset/region expressions.

* Add `distal_interval` and `proximal_interval` expressions that return a region defined by an initial locset and a distance to extend in the distal or proximal directions respectively.
* Add `radius_lt`, `radius_le`, `radius_gt`, `radius_ge` region expressions describing the subset of the morphology with radius less than (or less than or equal to etc.) a given value.
* Add `z_dist_from_soma_lt`, `z_dist_from_soma_le`, `z_dist_from_soma_gt`, `z_dist_from_soma_ge` region expressions describing the subset of the morphology which is less than (resp. less than or equal to, etc.) the given distance along the z-axis from the root sample.
* Add `uniform` locset expression giving a uniformly sampled set of locations from a given region based on a supplied PRNG seed value and a closed interval [left, right] of elements from the corresponding pseudo-random sequence.
* Add `Random123` as a library to `ext` and `util::uniform` wrapper for use in the `uniform` locset expression.
* Add unit tests to suit.

Fixes #939