Write cell/point data correctly for OPEs

Now that OPEs have a tuple degree, we need to change the tests for
point or cell data.
Add interval x interval in file output
Write correct connectivity for extruded quads

Vtk quad numbering is not our numbering, so we must reorder
the connectivity appropriately.
Speed up connectivity computation in extruded output

Rather than doing python loops over the whole mesh, build the
connectivity using some numpy primitives.  Reordering for quads is now
much simpler as well.
Merge pull request #180 from firedrakeproject/vtk-fixes

Fix bug in output of functions on extruded meshes and add
support for output on extruded intervals.