Adding new file to apply cohomology tests to extruded 1D interval.
Set boundary condition for Betti2 test.
Accept single value in extract_offset

In the 1D extruded case, the facet_map only contains a single value, so
we must convert to an iterable first.
Fix bc application for extruded cohomology test

It is not possible to pass a list of subdomains to the DirichletBC
constructor, so build a list of bcs instead.
Fixup lint warnings
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.