PredictionProblemGenerator make window_size optional

[PatrikDurdevic]

For the turbofan degradation example, the prediction problem should be:

• For each <unit_number> predict the number of records

With the current Trane, we have to specify window_size, so we can only get prediction problems like:

• For each <unit_number> predict the number of records in next <window_size> days

[PatrikDurdevic]

Realized this is accomplished with window_size=None