DiffusionIntegrator: implement 3D tensorized diagonal assembly
DiffusionIntegrator: implement 3D tensorized diagonal assembly

This is an ugly implementation, I'm sure the memory accesses are
terrible, but the asymptotic computational complexity should be
correct.

mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem