Calib: add model beam fractions for SCUBA-2

Add the fractional contributions from the ideal main and secondary
(error) beams to the total power within the SCUBA-2 beam. The hash
returned by the beampar method now includes the FRAC1 and FRAC2
entries, corresponding to the primary and secondary beam fractions.

A new method, beamfrac, returns both fractions by default or just the
named fraction only (err or main) for comparison with measured values.