Change set_port_status to be a Property

[MichaelWasher]

The set_port_status function in Chewie.py does not use properties to control access instead the value of port_status is public and there is also a public function intended to be used to drive the events. This can leave the state machines that are subscribe to fall out of sync with Chewie.

Change to a property and enforce a clean sub/observer pattern.