Assign area and owner for equivalent star bus from three-winding transformer

[jinningwang]

Is your feature request related to a problem? Please describe.
When three-winding transformer exists, there will be equivalent star bus created. Those buses do not have assigned area/owner/zone.

Describe the solution you'd like
Automatically assign the created bus area/owner/zone using the corresponding high-voltage side values.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Proposed solution:
A helper function named _check_star_bus(), go to System.setup()