text: Title: Is changing `Power` from an NpElemOperator to an NpPairOperator a good idea?
  Body: In order to achieve the expression like **"Power($close, Rank($close, 14))"** which is fairly common in fomulla alpha.
  
  I guess we could use following code:
  
  `class Power(NpPairOperator):
      def __init__(self, feature_left, feature_right):
          super(PairPower, self).__init__(feature_left, feature_right, "power")`
  
  Finally, we could add a __rpow__ magic method to **Expression** class