Add additional images to output

This MR adds the Stokes-I residual and model images to the output of Rapthor (previously only the Stokes I image was output). This work was done as part of RAP-197 as it makes it easier to interpret some of the results of runs on test fields (e.g., the residual image allows one to see if all emission was cleaned properly). Additionally, the sky models generated during imaging are also saved in work_dir/skymodels/image_X, as previously these sky models were not saved to the skymodels directory in the final selfcal iteration (and therefore it was somewhat difficult to find the final sky models produced by a run).