Added memory logging using 'ja' job accounting in PBS script.

If running inside a PBS script then 'ja' should print resource usage
info for the current job.
See
http://svn.nics.tennessee.edu/repos/pbstools/trunk/bin/ja

Unfortunately I think this implementation spawns a new fork
every time it tries to run the process via Popen, which caused it
to hang after a while.
Improved the PBS resource tracking.

Rather than calling the 'ja' script each time, causing a new fork
and clone (which caused me a freeze) we make a single subprocess,
if we appear to be in a PBS environment, and keep it running.
We also now do some processing of the PBS job resource information
and add it to the statistics.xls spreadsheet