Extend cable cell probe variety. (#1034)

Fixes #589 and #730, providing cell-wide probes and a correct total trans-membrane current.
Fixes #822, providing spatial interpolation of membrane voltages as governed by cable resistivity and diameter.

* Add partition of point mechanism target vector by cell index to `fvm_mechanism_config`.
* Fix sign error in axial current interpolation.
* Replace `fvm_probe_info` with class that wraps a variant type capturing one of a set of backend-cellgroup probe translation representations: `fvm_probe_scalar`, `fvm_probe_interpolated`, `fvm_probe_multi`, `fvm_probe_weighted_multi`, `fvm_probe_membrance_currents`.
* Refactor `fvm_lowered_cell_impl::resolve_probe_address` so that it avoids the long chain of if-else tests on wrapped `any` type with an invocation of `util::any_visitor`. Each specific cable cell probe address representation is translated into one of the intermediate `fvm_probe_info` variants via an overload of `resolve_probe`.
* Add new non-scalar cell pr...