Update error computations to support stochastic prediction

[nreinicke]

Once we have estimators that can provide stochastic energy estimates, we need to update the error metrics for these.

First, we'll need to identify an estimator that is capable of predicting in a stochastic manner. Perhaps the Estimator class should also require a method called prediction_type() that indicates if the model can predict stochastic estimates or not and then our error computation routines can compute different error metrics depending on the prediction type (PredictionType.DETERMINISTIC, PredictionType.STOCHASIC).

Second, we'll need to define a set of error metrics that make sense for the stochastic models. @ShashiKiran07 @jakeholden have you guys developed error metrics we should include here?