Made RETAIN_REFERENCE an inclusion of CUBEMEASURES such that it will only be...

Made RETAIN_REFERENCE an inclusion of CUBEMEASURES such that it will only be selectable when CUBEMEASURES is also selected.  This is because the option does not have any effect without enabling CUBEMEASURES.  This change is in reference to similar confusion with expected behavior related to issue #549.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3626 41f8697f-d340-4b68-9986-7bafba869bb8