* Replace old `arb::morphology` class with new `arb::sample_tree` and `arb::morphology` types, where `arb::sample_tree` manages a set of morphology sample points and their topology, and `arb::morphology` presents a branch-based view over samples described by an `arb::sample_tree`. The new classes support morphologies with cable-like or spherical somata.
* Supply a new conversion routine for forming a `arb::cable_cell` from a morphology, with support for both classes of soma.

Fixes #835