Changed to allow ranges of a single DN to fix #400. Also refactored code to...

Changed to allow ranges of a single DN to fix #400.  Also refactored code to reduce duplication, and updated stylistic elements (partially) to align with new standards.  Added new make command to test this new addition.  Updated documentation to include new functionality, and explain that all ranges are fully inclusive.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3142 41f8697f-d340-4b68-9986-7bafba869bb8