Separate forecast and cross validation

[CyGei]

get_fcast() is a two step process:

1. time series cross validation: models are scored with WIS
2. top_n models seclected + ensembling for forecasting h weeks ahead.

For future implementation with models that may be slow, we want to seperate those two steps:

1. becomes cross_validation() (or ts_cv())
2. get_fcast() may require providing the models explicitely.