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.
I am not sure what the syntax would be, but can one add a static assert to make sure this really is the correct type? Maybe something like https://stackoverflow.com/questions/17679268/enforce-template-type-through-static-assert/17680238#17680238 ? Do we do something like this anywhere in the library, yet?