Added updated approximation functions for Pixel/Detector/Line and Sample...

Added updated approximation functions for Pixel/Detector/Line and Sample resolutions.  Accuracy is greatly improved for nadir images.  Fixes #476

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@7064 41f8697f-d340-4b68-9986-7bafba869bb8