Arima : Make recipe reproducible and support gaps

[goldentom42]

Make Arima Reproducible

Arima does not use dates, so asking a prediction for the 10th of October or the 1st of October without pre-processing would lead Arima to provide the exact same prediction.

Arima transformer would need to keep track of the latest date seen by the model for each Time Group and pad the input query accordingly.

Make Arima support gaps

Same issue really. Need to pad for gaps before asking Arima to output predictions.