CAS-13960 : fullsummary is no longer a subparameter
fullsummary was exposed as a parameter to the user only when
`niter != 0`. Since this branch implements a return dictionary for the
`niter = 0` case, this has been changed such that `fullsummary` is
always exposed to the user.
CAS-13960 : Fix case of existing model image
In the event of an `niter=0` run with an existing model image on disk,
`tclean` would still report `modelFlux=0` from the return dict. This
commit implements the change to read the model flux from disk (if it
exists) and return the correct model flux, and returns zero if the image
does not exist.
CAS-13960 : Refactor of return dictionary class
Refactored the functions that generate the return dictionary, moved them
into a class, that live in a newly created module
`imagerhelpers/imager_return_dict.py`
This is in preparation for enabling the same functionality within the
`deconvolve` task.
CAS-13960: Enable return dict for niter=0 in deconvolve
Following on from the previous commit, niter=0 is now turned on in
task_deconvolve, reusing the same class that is called in task_tclean.
