Standard Models

Deals with #7. Looks like I'd already made the standard model. Renaming because we've used `make_` for things that return function closures and I feel like these final two functions `standard_model` and `float_model` are the main outputs for this module.

Added some comments to segment the file. Not sure how people feel about this. I definitely think having one models.py is nicer, as it's not a lot of code and mostly it's all used internally to compose these higher-level models so not really any point splitting it up. Comment segmentation is nice but perhaps a bit artificial.