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.
If you have to keep the `reinterpret_cast` because the direct assignment I outlined above doesn't go through the compiler, then what you really want to check in the `Assert` here is whether a `static_cast` *would have succeeded*. The predicate to test is `std::is_convertible` in C++11, see http://en.cppreference.com/w/cpp/types/is_convertible . Of course, we can't use that (yet), but you can use the replacement from BOOST: http://www.boost.org/doc/libs/1_36_0/libs/type_traits/doc/html/boost_typetraits/reference/is_convertible.html