fvOptions: Separate options for all cells, cellSets and inter-region coupling

by introducing rational base-classes rather than using the hideous
'switch' statement.  Further rationalization of the cell-selection
mechanism will be implemented via an appropriate class hierarchy to
replace the remaining 'switch' statement.

Mesh-motion is currently handled very inefficiently for cellSets and not
at all for inter-region coupling.  The former will be improved when the
cell-selection classes are written and the latter by making the
meshToMesh class a MeshObject after it has been corrected for mapFields.