Add "Fast & Frugal Trees" (AKA "fftrees")

[austospumanto]

Willing to help.

References:

• https://learncuriously.wordpress.com/2019/05/05/fast-and-frugal-trees-vs-machine-learning-trees/
• https://github.com/dominiczy/fasttrees
• http://journal.sjdm.org/17/17217/jdm17217.html
• https://ndphillips.github.io/FFTrees.html
• (Chapters 2 and 3) https://www.google.com/books/edition/Classification_in_the_Wild/HBkSEAAAQBAJ?hl=en&gbpv=0

[csinva]

Hi Austin, thanks for the link!

ftrees would be a very nice add to the package :) We are a bit busy at the moment trying to integrate this package with autogluon, but if you made a PR with this we would be more than happy to help review/integrate it into the package.

P.S. A little confusing but I think we would file fftrees as a "rule_list" rather than a "tree" since it really only has one path that goes down to lower depths.