HistogramTool.h: 2012-01-18 Steven Lambright and Jai Rideout - Fixed issue...

HistogramTool.h: 2012-01-18 Steven Lambright and Jai Rideout - Fixed issue where histograms were not created correctly for any bands but band 1. Added check for RGB mode. Fixes #668. 

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3887 41f8697f-d340-4b68-9986-7bafba869bb8