Create Class Properties for CLV _logp Methods

[ColtAllen]

ParetoNBDModel and BetaGeoBetaBinomModel use their respective logp functions for some of the predictive methods. Right nowlogp is being compiled every time a predictive method is called, which is inefficient. Caching this a class property would be an easy way to speed up predictions for these models.

[Ishaanjolly]

Any suggestions on how to do this? It does not seem basic to just add @cached_property decorator on a @static_class function...