Adds a folder for Turing models, and a few models for testing,

[JasonPekos]

Purpose

This PR adds a few Turing models:

• eight_schools_noncentered and eight_schools_centered. (chosen because they're classic)
• earn_height (chosen because the data needed for this model is a subset of the dataframe, re: https://github.com/stan-dev/posteriordb/issues/260))
• ness (chosen because the model has multiple associated posteriors, for different data)

Rationale

Eventually the rest of the Turing models are probably going to live here. For now, while I'm building out the Julia testing and benchmarking code, I'd like some models in the actual repository to test my code on.

These models cover some edge cases that I want to make sure my code covers.