Feature/generalize fvgridgeometry
What this MR does / why does DuMux need it:
So far this renames FVGridGeometry to GridGeometry for box, tpfa, mpfa and staggered.
** Things left to do:**
- deprecate FVGridGeometry property correctly
- open new MR with renaming and deprecation of all places where a public FVGridGeometry is used --> see !1700 (merged)
- discuss if a warning needs to be thrown and where (e.g. similar to the property-system warning
- merge !1700 (merged) after testing this branch thoroughly (this branch is how it will look like to a user, before taking care of the deprecation warnings)
Special notes for your reviewer:
This now only changes and deprecates the aliases for the fvelementgeometry.hh files, the renaming and deprecation of all other instances (in the tests etc) will be done in a new MR.
Fixes #719 (closed)