Take the CLV grid to the next level

[ricardoV94]

The idea is to follow up to #25, and extend the basic models in interesting ways, e.g., by adding hierarchical effects, time-varying effects... and so on. This should give us a more refined idea not only of what building blocks we need, but how flexible they should be.

This will, potentially, also be the biggest selling point of the package, as we will be doing things that are not really done out there (or at least not published in neat papers / packages), in large part by taking advantage of working with a fully-fledged PPL (PyMC!)

Unlike #25, these squares are not yet fixed, and any cool idea you have can be used.

• Continuous Non-contractual + Hierachical structure (Implement pre-built BG/NBD/BetaGeoFitter model #39) (up for grabs)
• Continuous Contractual + ??? (up for grabs)
• Discrete Non-contractual + ??? (up for grabs)
• Discrete Contractual + cohort / temporal effects (suggested to @drbenvincent), see Explore variants of the shifted Beta Geometric model #35

Possible the grid won't be over the 4 types of models, but perhaps over extensions:

• Cohort + Temporal effects on lifetime
• Cohort + Temporal effects on value
• Complex interactions between Lifetime and Value components
  • E.g., subscription fee affects churn-rate and value, @juanitorduz brought something that resonates with this
• THE NEXT BANG IN MARKETING MODELS ???