Turn matprep submodule into a workflow run from this repository

[sappelhoff]

The matprep repository by @a-hurst has been super helpful, but it also (understandably) hasn't received a lot of updates in the past: https://github.com/a-hurst/matprep_artifacts

I think for maintainability, we should:

1. copy over the functionality to this repository (have the matlab scripts under a "tools" directory or similar)
2. create GitHub workflows that run regularly, producing matlab PREP artifacts
3. use the regularly produced matlab PREP artifacts
4. remove the matprep git submodule

What do you think, Austin?