Adding prmon as an external
As we discussed in ATEAM-495 we'd like to enable prmon instead of MemoryMonitor in the Athena master branch. This MR is adding it as an external.
As this is my first contribution to the externals I most welcome comments/suggestions etc. I think it'd be better to install the source tarball to somewhere else (EOS?) so that we are not affected by possible network issues (trying to download from github). What's the best way to proceed w/ that?
Once merged, this one will close ATLINFR-2934.
Edited by Alaettin Serhan Mete