Define metrics for the quality of the simulation results

[phelps-sg]

It would be useful to quantify the quality of the simulation results. If we can measure how good the simulation is, those metrics could be used to determine whether the model is improving in quality as new features are implemented or as issues are resolved. In order to assess the performance of the model, it will be necessary to state the purpose of the model (#246) so that suitable performance metrics can be defined. If the goal of the model is to make forecasts, then the quality of the simulation outputs could be assessed by some kind of MSE metric (#206).