Fix bad param values in partial CV coverage. (#1009)

* Replace code that determines density mechanism parameter values across a CV with code that (hopefully) does it correctly. New code has the advantage also of being a bit simpler.
* Add unit test `fvm_layout.density_norm_area_partial` that catches the bug, and which validates the fix.
* Change old 'segment' terminology in `fvm_layout` test comments to 'branch'.

Fixes #1008.