Fixed log and corrected logical problem where program overwrote a base measure...

Fixed log and corrected logical problem where program overwrote a base measure if the new measure was the reference, when it should have been overwriting the base measure if the BASE was the reference and OVERWRITEREFERENCE=true.  This change is in reference to issues #229 #230.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2844 41f8697f-d340-4b68-9986-7bafba869bb8