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 a potentially hot function and a static variable is always surrounded by an `if` whether it is already initialized or still needs to be initialized. It probably doesn't matter very much for run-time, but one could avoid the if by moving this variable to a static member variable of the class.