Replace cable cell discretization. (#941)

Replace branch/cable-based cell representation and discretization with
free specification of control volumes over the cell morphology.

Implements functionality required for issues #859 and #732, and the
reworking of inter-CV conductance computation addresses issue #635,
although there is possible room for improvement by treating CV voltages
as averages rather than point estimates.

* Remove arbor/segment.hpp and segment-based representation in `cable_cell`.
* Add a new discretization policy `cv_policy_every_sample` that replaces
  the `compartments_from_discretization` flag in the `cable_cell`
  constructor.
* Remove use of `cable_cell::num_compartments()` in example code (see
  issue #940).
* Add command line argument to `single_cell` example to specify number
  of CVs per branch.
* Replace fvm layout `fvm_discretization` with new representation
  `fvm_cv_discretization`, which in turn wraps a `cv_geometry` object
  representing CVs as cables within cells in a cell group.
* Provide `cv_geometry_from_ends` that produces `cv_geometry`
  discretization for a cell given a locset comprising CV boundary points.
* Provide interface and functionality for merging discretization
  structures `cv_geometry`, `fvm_cv_discretization`, `fvm_mechanism_data`
  from multiple cells, in order to support (in the future) parallelized
  discretization across a cell group.
* Provide complete reimplementation of `fvm_discretize(...)`.
* Remove now redundant `arbor/fvm_compartment.hpp`.
* Add new locset expression `on_branches`, representing a point on every
  branch at a given relative position.
* Fix implementation bug in `pw_elements` `zip` function.
* Add `util::equal` range-based analogue of `std::equal`.
* Remove the compartments_from_discretization argument from cable cell
  made in python module. (Does this need to begiven a discretization policy?
  It is not clear.)