Add a periodic interval mesh

A horrifically hacky temporary solution for a periodic interval.  We
build a truly periodic mesh for Fluidity to read so that the
connectivity is correct.  Then, knowing the numbering that Fluidity
spits out for this mesh, we replace the coordinate field with the
appropriate DG field on the interval.
Correctly extrude DG coordinate field

In the periodic base case, the incoming coordinate field is DG, and we
should build a P1dgxP1 coordinate field for the extruded mesh (rather
than P1xP1).  To do so, we need to look at the number of dofs in the
base coordinate field, rather than the number of vertices in the base
mesh.
Add xfailing tests computing area of extruded interval

If we use a DG coordinate field for the base mesh and extrude it upwards
into P1dgxP1, computation of the area of the resulting extruded mesh is
incorrect (we never seem to get off the first layer).