Support for lifelines

[AbdealiLoKo]

Just thought I'd put in a Feature Request to support lifelines - https://github.com/CamDavidsonPilon/lifelines

It has some form of compatibility with sklearn.
https://lifelines.readthedocs.io/en/latest/Compatibility%20with%20scikit-learn.html

But after reading up on https://github.com/jpmml/sklearn2pmml-plugin I realized that it would need a java component for it to work.

[vruusmann]

Moved this issue back to the JPMML-SkLearn project, because preliminary analysis shows that there are not enough "interesting" classes in the lifelines package that would warrant setting up a separate Java project/library for that.

Also, my initial moves will be likely towards the scikit-survival package, which provides several estimator classes that can be approximated as regressors.

[vruusmann]

Cross-linking with #174