Enable list, save, restore, delete and rename flag version files.
These flag version files are copies of the flag column for a
measurement set. They can be restored to the data set to obtain
a previous flag version. On running importvla, a flag
version call 'Original' is automatically produced. It is wise to
save a flagversion at the beginning or after serious editing.
Keyword arguments:
vis -- Name of input visibility file
default: none. example: vis='ngc5921.ms'
mode -- Flag version operation
default: 'list'; to list existing flagtables
'save' will save flag column from vis to a specified flag file
'restore' will place the specified flag file into vis
'delete' will delete specified flag file
'rename' will rename a specified flag file
versionname -- Flag version name
default: none; example: versionname='original_data'
No imbedded blanks in the versionname
comment -- Short description of a versionname, when mode is 'save' or 'rename'
default: ''; example: comment='Clip above 1.85'
comment = versionname
oldname -- When mode='rename', the flag file to rename
merge -- Merge operation
Options: 'or','and', but not recommended for now.