ALMA-IMF II: Continuum data processing

ALMA-IMF continuum data.

Each field (sky position) has observations at two wavelengths, B3 (about 3mm) and B6 (about 1mm).  These are split further into:

* 'cleanest': continuum with minimized line contamination (and poorest sensitivity).  These are named like " W51-IRS2_B6_uid___A001_X1296_X187_continuum_merged_12M_robust0_selfcal9_finaliter.image.tt0.fits"; they do not have 'cleanest' in the name.

* 'bsens': continuum with maximized sensitivity (and maximized line contamination).  These have names like "W51-IRS2_B6_uid___A001_X1296_X187_continuum_merged_bsens_12M_robust0_selfcal9_finaliter.image.tt0.fits"

* 'bsens_noco' (B6) and 'bsens_non2hp' (B3) - 'bsens' data sets, but excluding the strongest contaminant lines in each band (12CO 2-1 and N2H+ 1-0, respectively).  These have names like "W51-IRS2_B6_uid___A001_X1296_X187_continuum_merged_bsens_noco_12M_robust0_selfcal9_finaliter.image.tt0.fits"

The images are all from self-calibrated data sets with variable numbers of iterations.

For each image, there are several filetypes:

* .image.tt0.pbcor: the Taylor Term 0 image with variable sensitivity and correct calibration (this is the closest to the true image of the sky).  Units [Jy/beam]

* .image.tt0: the Taylor Term 0 (flat continuum) image with uniform sensitivity (but incorrect calibration). Units [Jy/beam]

* .image.tt1: the Taylor Term 1 image

* .mask: the clean mask used in the final iteration of self-calibration

* .model.tt0 and .model.tt1: the model images in units of Jy/pixel

* .residual.tt0 and .residual.tt1: the residual images in units of Jy/dirty beam