Setting a new reference measure will now attempt to set the measure to not be...

Setting a new reference measure will now attempt to set the measure to not be ignored (which will fail if the measure if edit locked).  If no reference is explicitly defined for a point, then instead of simply choosing the first measure of the point as the implicit reference, the reference will implicitly be the first not ignored measure.  This will also take affect when adding new measures unless the point is locked.  The application pointreg will now explicitly set the reference to be the implicit or explicit reference it uses to register to.  These changes fix #274.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2926 41f8697f-d340-4b68-9986-7bafba869bb8