Merged ProcessImport, ProcessImportFits, SpecialPixel changes from prior...

Merged ProcessImport, ProcessImportFits, SpecialPixel changes from prior branch to allow import of 4byte SignedInteger images.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/osirisrex_2016-04-19@6698 41f8697f-d340-4b68-9986-7bafba869bb8