More Stats Functions

[ds26gte]

Issue brownplt/code.pyret.org#520 filed by @schanzer

We've had a few teachers ask if Pyret supports various stats functions:

• [x] population variance
• [x] sample variance
• [x] t-test (multiple kinds)
• [x] z-test
• [x] chi-squared test
• [x] multiple regression

Getting these implemented as a Pyret program would be great, but implementing them as part of Pyret's stats library would be much better.

(In keeping with the other stats functions, these should all operate on lists. I'll wrap them to work with tables in the DS teachpack.)