Add Additional Expressions to sBG Model

[ColtAllen]

UPDATED 2025-OCT-18

There is a follow-up research paper for ShiftedBetaGeoModel that introduces additional predictive methods:

https://faculty.wharton.upenn.edu/wp-content/uploads/2012/04/Fader_hardie_contractual_mksc_10.pdf

Specifically, expected discounted residual lifetime and expected retention elasticity.

[ricardoV94]

Probably relevant: #169

[ColtAllen]

I think adding customer_lifetime_value support for this model is a good idea, per #527.