casa6

Pull request #728: CAS-13085

Merge in CASA/casa6 from CAS-13085 to master

* commit 'e13ae7abcc887ef6197e92fceb4aab07bd0c6778': (26 commits)
  CAS-13085: as requested by Sandra, removed anachronistic imports
  Enable test_task_testconcat in Bamboo tests
  CAS-13085: Made virtualconcat raise an exception when an input MS is a link.
  CAS-13085: Added an improvement to the handling of missing scratch columns to virtualconcat which I had already introduced earlier in concat. Both tasks behave the same now. Deleted some commented out lines in concat.
  Removed casa5 related imports and any use of is_CASA6/is_python3 from the code.
  CAS-13085: extended mslisthelper method check_mslist by functionlity to verify the presence of unpopulated POINTING tables.
  CAS-13085: Added test for unpopulated optional Main Table columns
  CAS-13085: align testconcat and concat with virtualconcat:   always use testcontent=Fals in check_mslist, i.e. warn   whenever there is a potential problem even if the datacolumn   is not populated.
  CAS-13085: make virtualconcat complain then there are inconsistent columns in the input MSs even if they are not populated
  Moved the mslisthelper to src/private as suggested by Tak.
  CAS-13085: sanitize members of vis input parameter to remove trailing "/"
  Made test scripts import ctsys for casa 6. Made task scripts use os.scandir instead of walk for casa 6.
  Verified Ville's CASA6 version of task_testconcat with the space-only indentation and copied it on the CASA5 version.
  Replace tabs with spaces in Casa 6 task_testconcat.py
  Aligned the two copies of task_testconcat and made both Python2/3 compatible.
  Updated unit tests for test_concat.
  Updating the datapath to the testdata in test_testconcat. Made both casa5 and casa6 test scripts identical.
  CAS-13095 typo correction in a comment
  CAS-13085: Removed the line   from _future_ import print_function as requested by S. Castro.
  CAS-13085: Introduced the use of mslisthelper.check_mslist() also in task_testconcat
  ...