SCUBA2: tag current and previous responsivity images

Use tags to store the current and previous responsivity images (as
resp_new and resp_old respectively) rather than using the Frame object
uhdr. Then the uhdr RESP_NEW and RESP_OLD can be used as flags as
intended.
SCUBA2: remove unused code

The code defining a reference subarray and its associated image is no
longer needed now that a focal-plane reference is created as needed so
it can all be deleted.
PICARD: add note on only getting results at JAC/JCMT