Checks 8 builds (4 compilers, with/without netcdf)

Refactored run_tests.sh to make it easy to change which modules are loaded,
also finished adding settings for PGI and NAG compilers. At the very least, we
can now automate this portion of the testing, though I want to add a little
more detail (compiler versions, timestamp of last commit prior to testing, etc)
to the output before I turn it into a cron job.

Also, I added a time-stamp to the build directories -- this will ensure that
every time I run the test it will create a clean checkout without having to
wipe out the old checkouts (in case I want to investigate error messages).