class_docstr: Size the pairs trades by allocating capital to them.
  
  Args:
      * pct_of_capital (float): The percentage of current capital to allocate to new trades this timestep
code: def __call__(self, target):
      weights = target.temp.get("weights")
      if weights:
          trade_capital = (
              target.capital * self.pct_of_capital / float(len(weights))
          )
          for trade_name in weights:
              target.allocate(
                  trade_capital, child=trade_name, update=False
              )
          target.update(target.now)
  
      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 SizePairsTrades obj's underlying member method code with example usage: obj.__call__(target) and returns: True