Implement norm and errornorm

More Dolfin compatibility.
Pass cell_orientations to par_loop where appropriate

If the element has a contravariant piola mapping, the sign of detJ is
important.  In the embedded manifold case, this means we need to pass in
the globally determined cell orientations to the kernel.
Add mixed poisson on sphere test
Add mixed Laplacian on embedded square test

Solve the mixed Laplacian with inhomogeneous Neumann boundary conditions
on a unit square embedded in 3D.
Merge pull request #184 from firedrakeproject/cell-orientations

Pass cell orientations in to par_loops where appropriate (element
mapping is contravariant piola, gdim != tdim).