added hexahedral topology support to BasisFactory.
Added Zn Function and zn() convenience constructor for the function z^n, n an integer.
Made dofOrdinalsForSubcell resilient against the possibility that there aren't any.
Further work toward a single-element Laplace computation in 3D.
Merge branch 'master' of https://github.com/CamelliaDPG/Camellia