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

Merged ProcessImport, ProcessImportFits, SpecialPixel changes from osirisrex branch to allow import of 4byte SignedInteger images. Fixes #3906

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6703 41f8697f-d340-4b68-9986-7bafba869bb8