Changed to iterate over the pairing file to construct new PCK DB file as...

Changed to iterate over the pairing file to construct new PCK DB file as opposed to iterating over the SPK DB file.  This change was made so that the SPK DB file can contain extra, outdated kernels in spite of the fact that the pairing file produced by JPL gets constantly updated, with changes including removal and replacement of older kernels.  Consequently, the ISIS 3 data area may contain extra kernels lying around, in case certain users need these replaced/removed kernels, and as such the SPK DB file may contain extra SPK kernels.  The PCK DB file, however, will only contain entries in the pairing file.  In the past, this program was failing behind the scenes because the SPK file contained entries the pairing file did not.  This will no longer be an issue, as the SPK file should contain everything in the pairing file and perhaps more.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3628 41f8697f-d340-4b68-9986-7bafba869bb8