Convert grid transfer kernels to COFFEE ast

Also make them look more like FE kernels by using an iteration space for
the first (transferred to) argument.
Implement grid transfer on refined intervals
Test basic interval refinement
Test grid transfer operators on intervals
Add check for refinement of overlapped meshes

If we've already grown the halos, we can't refine the parallel mesh, so
add a sensible error message early and bail out.
Enable multigrid tests on travis