Changing control point type between fixed and constrained no longer gives a...

Changing control point type between fixed and constrained no longer gives a warning about the serial number not being unique and no longer crashes. Fixes #2060.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6197 41f8697f-d340-4b68-9986-7bafba869bb8