Moved duplicate point check for DUPLICATEPOINTS=ERROR to occur before merging...

Moved duplicate point check for DUPLICATEPOINTS=ERROR to occur before merging is attempted.  In this way, errors can be caught before expensive merging begins.  Also enabled logging of duplicate points.  If the user specifies a LOG file with DUPLICATEPOINTS=ERROR, all duplicate points will be reported to that file before the program terminates.  Fixes #545.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4245 41f8697f-d340-4b68-9986-7bafba869bb8