XDMF output for 2-, 3-tensor rank variables

[brryan]

#781 changed the HDF5 output such that all tensor rank variables are no longer recast as vectors. This means XDMF output needs to specialize to each tensor rank. With #820 this will be the case for scalar and vector data, but 2- and 3-rank variables will not be supported. This should be straightforward to add, and would also be a good opportunity to include 2- and 3-rank variables in the regression tests somewhere.