Fix of doc string for ExtrudedMeshes.
Make helmoltz and project extrusion tests shorter.
Fix docstring for make_extruded_coords
Merge pull request #43 from firedrakeproject/extrusion

Shorten extrusion Helmholtz / Project tests, docstring fixes.
Establish subsets for the boundary marks.
Cause external facet integrals to occur only over the relevant subset.
Minor bugfix to cause the correct subset index to be found. Introduce test of facet integrals.
Initial definition of the DirichletBC class.
Tell travis about my subsets dependency.
Some PEP8 pedantry.
Wire in the modified maps for external boundary conditions. Probably doesn't work yet but at least it doesn't cause any regressions.
Extend boundary condition support to interior facet maps and cell maps.
Put all the brackets into the tests now that cell_node_map is a method rather than a property.
Tweaks to cause the dirichlet conditions to actually be applied, although the dientification of boundary nodes is currently fubar.
More PEP8 idiocy.
Introduce code to correctly calculate the nodes involved in a DirichletBC, and cause those BCs to be correctly applied to matrices.
Register the parent maps of boundary condition maps in order to make error checking on sparsities work.
Add the ability to insert boundary condition values into a residual vector. This is achieved by adding the ability to evaluate a pointwise expression only on a subset.
Ensure the boundary conditions actually show up in the nonlinear solve, and deal with some whitespace pedantry.
Fix a couple more typos, and use union1d instead of unique.
After this change, nonlinear solves with strong boundary conditions appear to work.