Modified sumspice to continue if an end time is not available for an image. A...

Modified sumspice to continue if an end time is not available for an image.  A new end time will be computed based on the start time and exposure duration.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@7528 41f8697f-d340-4b68-9986-7bafba869bb8