Add polyhedron support to XDMF2
This is similar to !1255 (closed) except for XDMF2 instead of XDMF3.
Although backporting this feature to XDMF2 from XDMF3 is not ideal, this is the easiest path forward in the near future because of lingering issues with XDMF3. See my old mailing list post for details.