PICARD: explicitly request calibration units of BEAM

That makes it more obvious that the results must be obtained from maps
calibrated in mJy/beam.
PICARD: set the NOCALIB flag to false

Uncalibrated data should have a false NOCALIB flag so enable the files
to be calibrated in alternative units.
PICARD: allow calibrated data to be converted from ARCSEC to BEAM

This primitive can now deal with the situation where the data
currently calibrated in mJy/beam should instead be calibrated in
mJy/arcsec**2 and vice versa.