Added new function fetchCoefficients() that retrieves all coefficients from a...

Added new function fetchCoefficients() that retrieves all coefficients from a calibration file in a single PvlObject.  This function is intelligent enough to fall back to defaults if keywords do not exist for specific filters, cameras, etc.  References #779.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4202 41f8697f-d340-4b68-9986-7bafba869bb8