class_docstr: Identify pairs whose indicator exceeds some threshold and save them on temp.
  
  Args:
      * threshold (float): The threshold to use for the indicator between pairs
      * indicator_name (str): The name of the indicator data set
  Sets:
      * pairs
code: def __call__(self, target):
      t = target.now
      indicators = target.get_data(self.indicator_name)
      columns = indicators.columns
      signal = (
          indicators.loc[t, :].values.reshape(-1, 1)
          - indicators.loc[t, :].values
      )
      pairs = pd.DataFrame(
          signal, columns=columns, index=columns
      ).stack()
      pairs.name = "weight"
      pairs = pairs[pairs > self.threshold]
      pairs.index.names = ["sell", "buy"]
      pairs = pairs.sort_values(ascending=False)
      target.temp["pairs"] = pairs
      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 PairsSignal obj's underlying member method code with example usage: obj.__call__(target) and returns: True