Modify delay_calculator for cv

[zcaiElvis]

In cross validation, the left-out fold cannot contain data from the rest of the data. The weighted_past_counts for a particular data point in the left-out fold is currently calculated from all past observed_counts, including those that are in the rest of the data (that are not in the left-out fold).

We need a separate version of delay_calculator for cross validation, such that the weighted_past_counts for data in the left-out fold are calculated only from past observed_counts within the left-out fold

[zcaiElvis]

We will come back to this once cv_estimate_rt and the current version of delay_calculator are done and well tested