In ProcessMosaic, added check for count band when priority=average. The mosaic...

In ProcessMosaic, added check for count band when priority=average. The mosaic apps should fail when priority=average and there is no count band. In handmos, added an app test for the error check when PRIORITY=AVERAGE and the mosaic does not have a count band. Fixes #746.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5746 41f8697f-d340-4b68-9986-7bafba869bb8