Modified file prefix in jigsaw to add an underscore between the given prefix...

Modified file prefix in jigsaw to add an underscore between the given prefix and the output file names. Fixes #4309

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce@7108 41f8697f-d340-4b68-9986-7bafba869bb8