Add on-the-fly predictor_event creation for extracted events

[adelavega]

Recently, we changed PredictorEvents to only contain information for non-extracted Predictors, to prevent duplication for events in ExtractedEvents. For extracted Predictors, the timecourse is created on the fly in report or compile Celery tasks.

But for power users in pyns, there is currently no way to get the timecourse of the variables for extracted predictors.

We can add the logic used in report or compile to /api/predictor_events route for extracted predictors, and test if its performant enough.