class_docstr: Determine the relative weighting and leverage of the pairs trade
  
  Args:
      * weight( float ): The weight to put on the buy trade
  Sets:
      * weights
code: def __call__(self, target):
      target.temp["weights"] = {
          target.get_data("buy"): self.weight,
          target.get_data("sell"): -self.weight,
      }
      return True
  
imports: from copy import deepcopy
  from future.utils import iteritems
  from datetime import date
  import pandas
  import numpy
  import bt
label_desc: Use image node_id 2 for calling the WeighPair obj's underlying member method code with example usage: obj.__call__(target) and returns: True