pwkit/environments/casa/tasks.py: attempt to fix clearcal for CASA 5.x
pwkit/environments/casa/tasks.py: attempt to fix clearcal for CASA 5.x

Clearcal isn't compatible with the new vis iterator deal. This seems to be the
workaround. We target CASA 5.x specifically since I'd guess that the new
iterator will support initcalset in something like CASA 6.x

pkgw/pwkit