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 probe address types for all-of-cell values, including point mechanism state, density mechanism state, ion currents and concentrations, membrane voltage by CV cable, and total trans-mebrane currents. The latter are ultimately calculated by computing the net axial current fluxes in each CV.
* Add `util::any_ptr` field to the sampling function interface, pointing to constant-per-simulation-run metadata associated with a probe address. Underlying metadata type varies between probe types.
* Extend `simple_sampler` so that it optionally captures sampler metadata.
* Add special case to `simple_sampler` for vector-valued sample data, allowing it to translate from a pointer-pair range to a std::vector.
* Fix minor bugs in `util::variant`.
* Add to `mc_cell_group` the logic for translating each intermediate probe representation to a sampler callback invocation, reusing temporary scratch vectors across calls.
* Add unit comments to `embed_pwlin_data`.
* Add probe variety demonstration `probe-demo` to examples, covering most of the new cable cell probe classes.
* Add `util::any_visitor` runtime-type dependent dispatcher class.
* Add `util::overload` helper for generating overloaded functional objects from individual functions, lambdas or functionals.
* Extend probe unit tests in `test_probe.cpp` to cover new probe address types.
* Add unit tests for `any_visitor` and `overload`.