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.
Actually, can't you just do
```
const typename Triangulation<dim,spacedim>::line_iterator line = it;
```
? Iterators are mutually assignable but the conversion constructor will throw an exception if that makes no sense. In other words, the assignment is syntactically always correct but will fail at runtime if it is semantically invalid.