Refactor functional programming: change _calculate_avg_non_heating_usage to pass all variables

[ethanstrominger]

• Change
  _calculate_avg_non_heating_usage () =>
  to

avg_non_heating_usage = _get_avg_non_heating_usage (      
    fuel_type,
    avg_summer_usage,
    dhw_input.estimated_water_heating_efficiency, 
    dhw_input.number_of_occupants,
    stand_by_losses, 
    heating_system_efficiency
)

Part 1 - parameters

• Add parameters and pass parameters to _get_avg_non_heating_usage
• Change self. for any of these variables to get rid of the self.
  Part 2 - return variable
• Modify _get_avg_non_heating_usage to return a value
• Include the value in the return home