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 __init__(self, pct_of_capital):
      super(SizePairsTrades, self).__init__()
      self.pct_of_capital = pct_of_capital
  
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 1 to create a new SizePairsTrades object from inherited base classes: bt with example: obj = SizePairsTrades(pct_of_capital)