Implement the option to consider "activity cues"

[mmeierer]

Implement the option to consider "activity cues" to improve the prediction of PAlive of the Pareto/NBD model, see e.g., https://www.sciencedirect.com/science/article/abs/pii/S0148296313003019

Thus the right-censored time period get smaller compared to the traditional approach when only relying on transactions, i.e. between (1) (a) the time of the last observed purchase versus (b) time of the last observed activity and (2) the end of the observation period.