Add HDF5 tree writer test.

made changes for preparations to push to master; still need to finish documentation changes and line length changes

writing to disk.

Hopefully this will become a standard style for Xdmf development.
All spaces - 80 character line wrapping.

ENH: Add XdmfHeavyDataController base class and make XdmfArray use it.  Add XdmfHeavyDataWriter base class and make XdmfWriter use it.  This is in preparation to add additional heavy data formats.