Added RESTOLERANCE option enabling users to skip validation for registrations...

Added RESTOLERANCE option enabling users to skip validation for registrations whose measurements differ in resolution greater than the specified tolerance; references #622.  Added new tests to check that this new feature works properly.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4189 41f8697f-d340-4b68-9986-7bafba869bb8