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.
This is actually not necessary... triangulation already has one such static member, and it is exactly what is returned whenever an object has an invalid manifold id associated to it... I removed it.