Previously, each time a dataset was written to hdf5, the file was opened
and closed. This was implemented because inside the heavy data writer
we have no idea whether another dataset will be written, so we don't know
whether to keep the file open. After profiling large xdmf writes it was
determined that closing the file appeared to be a bottleneck so hooks were
added to the heavy data writer to open the file and keep it open while writing.
This is optional, but was incorporated into several higher level classes
like XdmfWriter where we know we can leave a file open until we finish writing
the XML. For our test problem we saw a 7 time performance gain.