Added support for exporting TIFFs greater than 2GB; fixes #579. Abstracted...

Added support for exporting TIFFs greater than 2GB; fixes #579.  Abstracted isis2std logic into a class hierarchy to support this new functionality.  Made necessary changes to ProcessExport to make the class hierarchy work, primarily adding a new Functor that takes a vector of Buffers.  Also removed duplicate code and imrpoved abstraction.  Added container class ExportDescription to encapsulate common export parameters from isis2std in a package that can easily be chained down to other processing objects.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4163 41f8697f-d340-4b68-9986-7bafba869bb8