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.