Tweaked.
Tweaked polCalc(), making consistent variable names, and commenting on the notes at the end about PA error as a function of sigma.
Added back in fwhm keyword in case data need to be smoothed in the INVERT step.  Also increased the CLEAN cutoff for the dust maps to 4*rms_theo, consistent with do_slmap().
Corrected error that had not allowed for restoring of maps with the fitted beam shape.  Also, added back in fwhm keyword in case data need to be smoothed in the INVERT step.