Disallow coarsening if any children of the parent cell are FE_Nothing

[stvdwtt]

Context: With AMR activated we can add material in refined areas that corresponds to some but not all of the child elements of a element in the coarse mesh.

Current behavior: As the heat source moves away from the area, the code tries to coarsen the mesh in the deposited region and ends up with NANs as it tries to merge active and inactive elements.

Desired behavior: The mesh stays refined if any other children of the parent cell are FE_Nothing.