Pull request #1028: CAS-14732
Merge in CASA/casa6 from CAS-14732 to master
* commit '4367509f637e844d0ace77ea151e165bd67efc38':
moved the check but had to fix stuff as empty string was acceptable at that stage.
move the check for units in starting model to SyntheisisImager
force saving of model image
Throw an exception if model is not in units of Jy/pixel
Fixed an image copy with mask being applied which did not copy the units
