CAS-13872 : Enable mt-mfs in task_deconvolve
The changes made in 8caf97da75b0a68fb and prior commits to fix the minor
cycle residual calculation in mtmfs have been checked, and the
corresponding MT-MFS tests in `task_deconvolve` have been enabled.
The deconvolve XML has been edited to remove the warning related to
MT-MFS, and the full description put in (from task_tclean).
The warnings in `task_deconvolve.py` have also been edited to allow
usage of MT-MFS.
