Add a mask channel index to fix WSClean !352
Non-joined deconvolution with a fits cube no longer worked in WSClean because of changes in https://gitlab.com/aroffringa/wsclean/-/merge_requests/352. An integration test for this is added in https://gitlab.com/aroffringa/wsclean/-/merge_requests/503.
In a later MR, the new variable should be exposed in the Python API.
Edited by Andre Offringa