use quantile with a 0.5 alpha to get a prediction of the median

[ClimbsRocks]

we can use the delta between the median prediction and the rmse prediction to do all kinds of interesting things...

for instance, it's one measure of uncertainty.

for uncertainty, we might end up with multiple definitions:

1. prediction_intervals
2. median prediction vs ls (rmse) prediction
3. predicted_uncertainty

[ClimbsRocks]

i think the returned values will be prediction_lower, prediction_upper, prediction_median, and prediction