In findgaps, added the option to nullify a gap and add a buffer of null lines...

In findgaps, added the option to nullify a gap and add a buffer of null lines above and below each gap and output these results to a cube. If no output cube is specified results will only be output to a log file (.pvl, .prt, or .txt) or vice versa. Neither output affects the other. Fixes #582.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4533 41f8697f-d340-4b68-9986-7bafba869bb8