Merge pull request #24 in CASA/casaviewer from CAS-13252 to master
* commit '8429ab68c091b959883f3ff8bcaf2d165b428b6c':
update casaviewer to updated casa6 branch (CAS-13252)
Revert "Revert "CAS-13252 update to pull the latest version of casa6 source, don't include files that were removed in casacore (Mutex.cc)""
CAS-13252 update to pull the latest version of casa6 source
CAS-13252 update to pull the latest version of casa6 source