Merge pull request #454 in CASA/casa6 from CAS-13540 to master
* commit '72d9ea87b1f8eb00b92a0cb2f06d5432f552a474':
Reset .gitmodule formatting in CAS-13540
update casacore submodule to use main casacore repos, not dmehring's
use commit on trunk casacore
AS-13540 support microsecond precision for obs-date in imhead