Add xdmf polyhedron support
This adds support for Polyhedron cells to Xdmf3
Elements of type Polyhedron (i.e. N face cells, where each face is a M edge polygon) are in the following format: [nCellFaces, nFace0Pts, id0_0, id0_1, ..., nFace1Pts, id1_0, id1_1, ..., ...]
An XDMF example file containing 2 polyhedron and 2 hexahedron cells is attached together with two python scripts that test the conversion from .xmf to .vtu and back. polyhedron.xmf testXdmfToVtk.py testVTKtoXdmf.py runTest.sh