move models to JSON files

[jamestwebber]

This should wait until after #198 as it was branched off from there.

I was working on building a custom model for some analysis and decided to write a PR to do something I believe @kvg had mentioned before--moving the pre-configured models into separate json files rather than hard-coding them in the model_utils module.

There are some pros and cons here:

• the file is a lot shorter 👍
• but of course there are a bunch of new files 👎
• it's easy to stick new models in that folder 👍
• using code meant constants to avoid typos for some tags, can't do that 👎
• json doesn't like to have comments in it 👎

Also not totally sold on the organization of where I stuck things...I didn't want to bury it too deeply, but it should be inside the package so that it's properly installed and accessible by importlib.resources.

[jamestwebber]

After some thought, tweaking this to flatten the json directory into just models and specifying array vs cdna in the JSON itself (which allows people to use one file for both, as in the tutorial).