Fix bc application in 1d nonperiodic cohomology test
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.
Use ExtrudedSet everywhere appropriate

Now the extruded sets are a first class citizen, we must use them
everywhere appropriate.  Additionally, remove unnecessary passing of
layers around to constructors (it's already in the mesh).
Extrude coordinates cell-wise not vertex-wise

Now that we can walk over "mixed" base and extruded meshes, produce
extruded coordinate fields cell-wise.  This allows us to write kernels
to extrude general base coordinate fields into base x P1 extruded
fields.
Stash measures on Mesh objects and use them in project

If we've built more than one mesh, we can't us a bare ufl.dx in project,
since it may pick a coordinate field not associated with the function
space in question.  To avoid this, save measures on the mesh object and
use them when constructing the form.
Add tests on extruded interval with DG coordinates

Compute the area of an extruded interval with a DG coordinate
field (both periodic and non-periodic cases).
Compute cohomology for periodic extruded interval

Add tests computing cohomology for a periodic interval (results the same
as the flat 2d annulus).
Fix extruded mesh "off by one" layer construction

The constructor to ExtrudedMesh now considers the layers argument to
count the number of cell layers (1 or more).  We add error checking of
the argument and then immediately translate to PyOP2 style
counting (repetitions of the base set in the extruded set).  Update all
tests to follow this change.
Correctly default layer_height if not provided

Previously even though we had code to add a default layer height it was
never reached because we errored beforehand if no layer height was
provided.  Switch this check to ensure we pass either a kernel or a
preset extrusion type, and update docstring appropriately.
Merge pull request #104 from firedrakeproject/1d-periodic

Add a periodic interval mesh with DG coordinate field, and the ability
to extrude such meshes.  Additionally, fix layers numbering bug in
user-facing API.
Bump version to 0.10.0