isis2pds and hidtmgen will no longer crash if the keyword MaximumLongitude...

isis2pds and hidtmgen will no longer crash if the keyword MaximumLongitude does not exist in the Mapping group label. Fixes #947.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6298 41f8697f-d340-4b68-9986-7bafba869bb8