Inverted Inner Product matrices on Curvilinear Mesh

[jcapriot]

The inner product matrices on CurvilinearMesh all require a full matrix inverse, as none of them are actually diagonal.
This means that
mesh.get_face_inner_product(invert_matrix=True)
should fail on CurvilinearMesh.
Currently it silently proceeds in a manner that is incorrect; it takes the inverse of the diagonal of the inner product Matrix.