Retrieve probe metadata from simulator object. (#1101)

* Add `get_probe_metadata` method to `simulator` that forwards probe metadata queries to appropriate cell group, via new interface method in cell group base class.
* Replace `gid_to_local_` map in simulator implementation with a map that takes gid to local cell index _and_ local cell group index.
* Move scheduler association map mutex into the cell group where it is being used (i.e. `mc_cell_group`).
* Add generic (`visit`-able) interface to specifc fvm-probe types for accessing metadata via `any_ptr`.
* Implement (override) `get_probe_metadata` method for `mc_cell_group`.
* Use new cv policy API for LFP example discretization.
* Use new probe metadata API to simplify LFP example sampler callback.
* Add `on_components` locset expression that gives points a relative distance from the head of each component of a region.
* Simplify implementation of thingification of `ls::boundary`.

Fixes #1079