[WIP] distribution classes

[pavanramkumar]

closes #350

TODOs

• [x] make it work for all distribution classes
• [x] add tests for BaseDistribution class
• [x] make tests and examples pass locally
• [x] remove unused hidden methods from pyglmnet.py (currently commented out)
• [x] migrate simulate_glm() functionality into simulate() methods of respective classes
• [ ] migrate metrics definitions that depend on log_likelihood into distribution classes (pseudo_R2, deviance)
• [ ] add an example to show how users should roll out their own distribution class (e.g., using jax numpy)
• [ ] ensure Probit and Binomial work correctly
• [ ] check if all docstrings still make sense
• [x] update API reference
• [ ] update what's new