Changed CONVEXHULL option to LOWCOVERAGE to more accurately describe how it is...

Changed CONVEXHULL option to LOWCOVERAGE to more accurately describe how it is useful, not so much what it does to accomplish its goal.  Also added output to the print out logging how many images failed this check with a prose description of the failure.  These changes reference issue #245.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3024 41f8697f-d340-4b68-9986-7bafba869bb8