SCUBA2: tag flatfield solutions with flatsol
SCUBA2: proceed if flatfield solutions exist

Check for the new flatsol tag before proceeding.

Also, POD updates.
SCUBA2: set tag/product to flatfield

The new tag/product indicates that Frame contains flatfielded
timeseries data.
SCUBA2: check for flatfielded data before proceeding
SCUBA2: tag raw fastflats for later retrieval
Merge branch 'tagflat'