Allow CV policies to be composed over subregions. (#1078)

* Move `cv_policy` classes to new source files.
* Remove `cv_policy_flag::single_root_cv`.
* Allow all supplied `cv_policy` makers to be restricted a domain specified by a region: the policy is then applied per component, with the boundary points of the completion of the components included.
* Expose `cv_policy` domain in public interface.
* Add `components` function for splitting an extent into disconnected components. Add comment describing what is meant by connected in this context.
* Rename `super` region function to `complete`.
* Add region expression functions `complement` and `difference`.
* Add new locset expression function `boundary` which returns the extrmal points of each component of a region.
* Add new locset expression `cboundary` which returns the extremal points of the completion of each component of the region, which is what we need to close off CVs generated by region-restricted CV policies.
* Add locset expression `supp...