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.
Reading the documentation of this function, I was confused how the function is supposed to determine whether it's a line or a quad until I realized (i) it doesn't actually need to care, (ii) one could infer this from the size of arguments (as the documentation states once one understands the first point).

Maybe rephrase the second sentence as
"The function needs to return one or multiple points (depending on the number of elements in the output vector @p points that lie inside a line, quad or hex. Whether it is a line, quad or hex doesn't really matter to this function but it can be inferred from the number of input points in the @p surrounding_points vector."