Changed computation of number of bands to work on to no longer rely on...

Changed computation of number of bands to work on to no longer rely on averaging the number of values for each keyword in the BandBin group.  Now takes the minimum range for the input and output cubes instead.  This change fixes #375.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3105 41f8697f-d340-4b68-9986-7bafba869bb8