Make micromag elements refineable
Use refineable elements to handle Neumann singularity in sq_mumag4

Add an extra node using refinement to mesh "sq_mumag4", we can then pin
phi1 on this node to prevent a singular Jacobian.
Cleaner way to avoid deleting nodes from bem mesh
In ArrayInterpolator use new has_hanging_nodes() function
In factory construction of "sqt_mumag4" clean up better

(Delete Qmesh after conversion)
Sketch of implementation of bem handler with pinned nodes