Fix Mesh creation when there are no interior facets

Single element meshes (such as the unit triangle and unit
tetrahedron) have no interior facets.  Unfortunately, we can't build a
memoryview on a C array of zero length, so guard against this case and
build an empty _Facets object instead.
Test interior facet integrals on single element mesh

A single element mesh has no interior facets, so assembling a form over
the appropriate measure should give zero.
Fix generation of circle meshes

Previously we were extruding the circle twice, resulting in a mesh with
twice the area we actually want.
Add tests of mesh generation

Check that building a mesh and assembling the measure over it gives the
appropriate "volume" of the mesh.
Merge pull request #18 from firedrakeproject/test-meshes

Test mesh creation:
* Fix Mesh creation when there are no interior facets
* Test interior facet integrals on single element mesh
* Add tests of mesh generation