Refactor to use functional programming for _calculate_avg_summer_usage

[ethanstrominger]

To do this, all references to self.attribute or cls.attribute need to be removed. Any attributes that are set must be passed back in the return value to the calling function.

• Change
  _calculate_avg_summer_usage() => avg_summer_usage = cls_get_avg_summer_usage(bills_summer)
  Do in two steps.
  Part 1 - avg_summer_usage
  • rename _calculate_avg_summer_usage to _get_avg_summer_usage
  • change self.avg_summer_usage to avg_summer_usage
  • return avg_summer_usage
  • change _calculate_avg_summer_usage()=>avg_summer_usage = cls_get_avg_summer_usage()`
  • at the end of the initializing function, add self.avg_summer_usage = avg_summer_usage
    When all functional programming is done, you can figure out how to do this differently
    Part 2 - bills_summer
  • Change self.bills_summer to bills_summer
  • Add bills_summer as a paramter to _get_avg_summer_usage and pass it when the function is called