CAS-13818 : Fix mask cache conflict on tclean restart
As documented, the fix for the original issue triggered a mask cache
conflict on restart. In the end the fix turned out to be fairly simple -
check if the mask exists and is valid, and do not touch it if it is.
So in the first instance, when the mask does not exist, it will do the
right thing. And it will not try to do anything on the restart either.
It ended up being a simple inversion of the original logic, with
appropriate traps in place so it works for both the original bug and the
derived bug.
CAS-13818: Add test to check for full stokes mask
Added a check within test_stokes_mtmfs_IQUV within test_task_tclean to test the
mask when using mtmfs + full Stokes.
CAS-13818 : Disable test_imgval_startmodel_axesmismatch on MacOS
Added a skip to test_imgval_startmodel_axesmismatch in test_task_deconvolve for
MacOS. This test fails for unknown reasons, unrelated to the code on this
branch.
The failure and solution are being investigated on CAS-14117 and will be
re-enabled on that ticket.
CAS-13818 : Fix for failing test
The previous commit fixed the issue from the parent ticket (no mask for
multi-stokes MT-MFS images) in the wrong location, causing a single tclean test
to fail. This commit moves the fix to the right spot, i.e., into
SDAlgorithMSMFS::restore which works.
All the tclean + deconvolve tests now pass locally.
