Fixed bug in get_group_coordinator()

The get_group_coordinator() method returns ‘None’ for a zone which is in Stereo mode, when invoked on a
device which is a slave.

The method also needs to be invoked on a device that is initialized with an IP address that belongs to the zone. This should not be the case.

I’ve refactored the method to get the topology of all devices and extract the coordinator for the given zone name. This allows us to get the coordinator IP for any zone from any initialized SoCo object.