message: fixes pick_up_tip() pressig too many times
diff: @@ -998,8 +998,6 @@ class Pipette(Instrument):
  for i in range(int(presses) - 1):
  self.robot.move_head(z=tip_plunge, mode='relative')
  - self.robot.move_head(z=-tip_plunge - 1, mode='relative')
  - self.robot.move_head(z=tip_plunge + 1, mode='relative')
  self.robot.move_head(z=-tip_plunge, mode='relative')
  _description = "Picking up tip {0}".format(
  