HDF5: Basic implementation of HDF5 output

The _HDF5File sets the timestep and default section on the Plex and
uses DMView and VecView to output function data.
HDF5: Generate Xdmf header when closing HDF5 file

This is done by invoking PETSc's post-processing scipt.
HDF5: Adding capability to output HDF5 for VectorFunctionSpaces

Instead of deriving the global node numbering from sections with
one DoF per node (vdim=1) we now derive the correct sections and
divide by the number of DoFs per node (vdim) to get node numbers.