Made MappingQ aware of Manifold

This is the last step that was necessary for the Manifold branch. 

With this modification, MappingQ, when constructed with the optional flag `curved_interior_cells`, will obey what manifold says for all interior support points. 

This version is fully compatible with the old Boundary classes.
OBJ is an iterator (cell, line, ...), right? Maybe rename it to ITER and document this?