message: add head & instrument to position tracker
diff: @@ -201,8 +201,10 @@ class Robot(object):
  self._previous_container = None
  - self._deck = containers.Deck()
  self.position_tracker = position_tracker.PositionTracker()
  + self.position_tracker.create_root_object('head', 0, 0, 0)
  +
  + self._deck = containers.Deck()
  self.setup_deck()
  self._instruments = {}
  @@ -240,6 +242,8 @@ class Robot(object):
  axis = axis.upper()
  self._instruments[axis] = instrument
  + self.position_tracker.track_object('head', instrument, 0, 0, 0)
  +
  def add_warning(self, warning_msg):
  """
  Internal. Add a runtime warning to the queue.
  