create metrics module

This module would implement forecasting metrics such as MAE, MBE, RMSE, error distributions, and more.

See 

Zhang et. al. "Metrics for Evaluating the Accuracy of Solar Power Forecasting", NREL/CP-5500-60142 (2013)

Perez et. al. "Comparison of numerical weather prediction solar irradiance forecasts in the US, Canada and Europe", Solar Energy 94 305 (2013)
