Fixed bug in Gain2x2 which only smoothed the real part of the YY^H term.
Fixes ratt-ru/calibration-pipelines#1, and also slightly improves subtraction
of other sources.
Added stefcal.make_diffgain_plots, and invoke it from stefcal.stefcal.
Merge branch 'devel' of github.com:ska-sa/meqtrees-cattery into devel