gmsh mesh-convert tests rely on facet numbering

[Prof Garth Wells]

Some gmsh tests in test_mesh_converter.py rely on specific numbering of facets in a mesh, but DOLFIN provides no guarantee on the numbers.

A new algorithm for numbering facets breaks these tests. The test code is cryptic. Function names and descriptions should be improved.

[Anders Logg (Chalmers)]

Removing milestone: 1.7 (automated comment)