Added new REVERSE option allowing user to treat last item in CLIST as base,...

Added new REVERSE option allowing user to treat last item in CLIST as base, this fixes #180.  Also added new test to check that this new feature indeed works.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3019 41f8697f-d340-4b68-9986-7bafba869bb8