Changed construction model back to in-place modification of the input SPK DB...

Changed construction model back to in-place modification of the input SPK DB file to preserve relative ordering of SPKs in the SPK DB file with the generated PCK entries in the PCK DB file

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3649 41f8697f-d340-4b68-9986-7bafba869bb8