update to internal accounting for finite-area

Changes the internal storage location of finite-area from the polyMesh registry to a dedicated registry. This allows a clearer separation of field types without name clashes (eg, can now have "U" as a name for an area velocity field). It is also a prerequisite to supporting multiple finite-area fields in the future.

Example of old locations:

0/Us
constant/faMesh
system/faSolution

New locations:

0/finite-area/Us
constant/finite-area/faMesh
system/finite-area/faSolution

The updated locations are handled by the foamUpgradeFiniteArea script, which provides a simple and transparent means of managing the upgrading without adding in multiple layers of compatibility handling within the code.