New peak error metrics

[praftery]

Include an error metric that looks at the peak month prediction. Utility bills for large buildings often charge based an overall $/kWh consumed (which we cover well with existing error metrics) as well as a $ per peak kW consumption in a month. These peak demands often make up half the total cost.

As yet, none of the error metrics evaluate model accuracy for this - and it can have a very significant impact on the end result.