Write a draft of the rabbit_mesh_geom function and a test for some of it

Soon, check for edge and face correctness using rabview. But first, it
should be necessary to change edges to be keyed by their rnp instead of
their vertices.
Switch to using rnp as key for edges, and using the geom function

There will be some code cleanup to do after this. In particular, the
rabfile and rabview programs are out of date.