Include pyop2_geometry.h md5 hash in FFC kernel cache key
Add compute_{min,max}_facet_edge_length_tetrahedron_3d
Merge pull request #283 from OP2/compute-facet-edge-length

Add compute_{min,max}_facet_edge_length_tetrahedron_3d

These functions are required in FFC kernels to compute normals in 3D.

Also add an md5 hash of pyop2_geometry.h to the FFC kernel cache to
make it a little more robust.