Merge pull request #285 in CASA/casa6 from CAS-13153 to master
* commit '28f5e9bbc5d5e5310a7ecffa8c441ad29900fc24':
CAS-13153 : Added a unit-init for the model image too.
CAS-13153 : added an import for impbcor, needed for casa6
CAS-13153 : Fixed bug where Jy/beam units were not making it to the output image if it had more than one stokes plane, with mtmfs. Added check for units into an existing verification test for this usage mode.