Stash a vertex numbering on the mesh
Slightly simplify logic in DG cell_node_map computation
Build cell_node_map for P1 FunctionSpaceHierarchy

Horrifically slow Python implementation for now.  The map you get is
isomorphic to the P2 map on the coarse mesh.
Add prolongation of P1 fields on triangles
Add more prolongation and restriction tests
Create correct coarse to fine cell map

Slow python version for now, but working.