Changed Control Point to no longer attempt to set a measure to un-ignored when...

Changed Control Point to no longer attempt to set a measure to un-ignored when explicitly set to be its reference.  The assumption here was likely that a reference should never be ignored, but there are plenty of examples (particularly when dealing with bundle adjustment) where there are good reasons for references to be ignored.  This change fixes #380 as that bug was caused by cnetmerge merging over an ignored measure and explicitly setting it to be the reference, which of course caused the Control Point to attempt to un-ignore the measure.  This problem is resolved by no longer having Control Point set references to be un-ignored, which will preserve the ignored status of references as implied by the OVERWRITEREFERENCE option of cnetmerge.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3118 41f8697f-d340-4b68-9986-7bafba869bb8