ENH: extend hist.civis_ml() to support multi-output models

[nik-bladey]

hist.ml() currently makes it easy to plot a histogram of the predicted scores or probabilities for single-output CivisML objects. I'd like to expand this function so it can also support multi-output.

[patr1ckm]

This is currently blocked because the predicted scores are not provided automatically for multi-output models by CivisML.