Simplify `pp.inference` API

[shwang]

The pp.inference API is a mess. All but two of the dozen or so functions available were used for experimentation/troubleshooting and are irrelevant for the paper, so it might be a good idea to scrap stuff for clarity.

For example, there's "gradient-descent to get MLE beta" code and an entire "hardmax/" folder, which is superfluous now that the "softmax" maxent inference has been removed from the repo.

API rework route:

• [ ] Maybe: pp.inference.hardmax => pp.inference
• [ ] Get rid of all the inference tools except those we actually use in Andrea's crazy fly repo.
• [ ] Submit PRs to all the repos that depend on this to install from pypi instead so they can use the legacy interface (pip install pedestrian-prediction == 0.1)
• [ ] Maybe write a note on the README.md for people who are referencing crazy fly