Just write the mesh files for unit tets and triangles

Since gmsh seems to be incapable of meshing a triangle into a single
triangle (version dependent), just build the mesh file directly, rather
than writing a geo and then meshing it.
Add internal facet integral test for unit tet
Merge pull request #62 from firedrakeproject/unit-triangle-issues

Since gmsh doesn't seem to want to produce single element meshes in some
cases, just hard-code the mesh file for unit triangles and tets rather
than relying on mesh generation.