code: def rpm(self, vol_per_rev):
          
          return Stock.rpm(self, vol_per_rev, self.Q_stock()).to(u.rev/u.min)
docs: Return the pump speed required for the reactor's stock of material
          given the volume of fluid output per revolution by the stock's pump.
  
          :param vol_per_rev: Volume of fluid pumped per revolution (dependent on pump and tubing)
          :type vol_per_rev: float
  
          :return: Pump speed for the material stock, in revolutions per minute
          :rtype: float