Implementing the `has-turret` trait

The has-turret trait is formally defined by YEP-304.

Implementing has-turret involves writing getter and setter methods, and storing one state variable:

    def set_turret(self, index):
        self._busy = True
        # Perform the actual setting of the turret for your device
    self._state["turret"] = index

    def get_turret(self):
        return self._state["turret"]

built 2020-09-26 00:52:26                                      CC0: no copyright