SCUBA2: report both FWHMs for two-component fits

And make sure this primitive is the only place where those beam
properties are reported - previously _FIT_SOURCE_ was also reporting
its results which were not always good.
SCUBA2: make sure product is restored to input setting

The first call to _CALC_SCUBA2_SNR_ sets the product to "snr", which
was only reset if the product on entry was "matched-filtered". Make
sure the input product is restored for all cases.
SCUBA2: force current Group object to be queried in fit

This forces _FIT_SOURCE_ to query the current Group object for tags,
products etc. The default is to query the current Frame, which can
lead to incorrect comparisons and the consequent decisions.
SCUBA2: reformat statements for clarity