Added feature in pixel2map for using an FOV based on the full range of the...

Added feature in pixel2map for using an FOV based on the full range of the exposure time. Fixes #4476. Fixed bug in VariableLineScanCameraDetectorMap that was calculating values on the middle of the line instead of the top. Fixes #4508.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@7207 41f8697f-d340-4b68-9986-7bafba869bb8