Provide ability to disable purge behavior of UFL interface

The PI Interface for UFL includes a PURGETIME setting to purge successfully processed files (_OK) after some period of time. We have a case where we do not want to purge processed files. We want the data to be in PI but want to preserve the original files in case there are issues. There are cases where the interface can successfully process files and send the data to the PI Data Archive. But the PI Data Archive has an error storing the data and so the data is effectively lost. Keeping the processed files around would give the ability to resolve the issue and reprocess the file. Rather than setting a very long purge time or creating a mechanism to copy processed files, elsewhere we would like to simply be able to disable purging. For example, set the PURGETIME to 0 or provide another flag to override purging.