Accelerated Failure Time Models

[pzivich]

Is your feature request related to a problem? Please describe.
Right now, only the Weibull accelerated failure time model is supported. If we implement the generalized-gamma model, then I will be able to implement various other distributions automatically (weibull, exponential, gamma, inverse-weibull, inverse-gamma). This could replace much of the survival.py estimating equations.

Describe the solution you'd like
Implement the generalized-gamma AFT model.

Describe alternatives you've considered
Bespoke estimating equations, which would be burdensome to implement and upkeep.

Additional context
Some references

https://www.sciencedirect.com/science/article/abs/pii/S0167947316302390

https://link.springer.com/article/10.1007/s00362-022-01300-4

https://univ-pau.hal.science/hal-02953269/

https://www.hindawi.com/journals/mpe/2011/150294/

https://lifelines.readthedocs.io/en/latest/fitters/regression/GeneralizedGammaRegressionFitter.html