[ENH] simple tabular prediction reduction for forecasting

[fkiraly]

This adds a simple tabular prediction reduction forecaster to the reducers module.

The new YfromX forecaster uses tabular regression to forecast y from temporally concurrent X - it does not lag y and just uses X at the same time.

This is perhaps the simplest reduction strategy to supervised tabular regression, and so far it has not been easily obtainable afaik.