New cable representation for CVs. (#994)

* Replace CV cable representation with one which makes it clear to which CV a fork point 'belongs': every fork point belongs to exactly one CV; if a CV representation has cables on two different branches at a fork point, it must have cables on all branches at that fork point, and the fork point is associated with that CV.
* Rework (and simplify) `cv_geometry_from_ends` to suit new representation.
* Add CV child information to `cv_geometry`.
* Add computed pw-constant axial resistivity over cell branches to `fvm_cv_discretization`, so it is available for voltage/current interpolation.
* Extend `location_cv` method: add a new parameter of enum `cv_prefer::type` which instructs `location_cv` on how to resolve locations on CV boundaries.
* Provide pw-constant over cable integration methods for `embed_pwlin`.
* Split out `unique_in_place` to its own header, with unit tests.
* Unit tests to suit.