Begin to address Issue #28
Begin to address Issue #28

Properly segment out the CV sample and exogenous samples for
out-of-sample scoring during fit. However, how to handle the situation
where forecasts will pick up at t_{n_samples - n_cv}?

TODO: fit on (n_samples - n_cv) to get the parameters, then assign full
y vec within the model as the next point for forecasts.

alkaline-ml/pmdarima,tgsmith61591/pyramid,alkaline-ml/pmdarima,tgsmith61591/pyramid,tgsmith61591/pyramid,alkaline-ml/pmdarima