AutoReg subpixel alogrithm now works for minimum difference and any other...

AutoReg subpixel alogrithm now works for minimum difference and any other lower-is-better matching algorithm; references #993

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4553 41f8697f-d340-4b68-9986-7bafba869bb8