Format yaml files

For example, using https://github.com/google/yamlfmt or https://lyz-code.github.io/yamlfix/ (these both seem more maintained than https://github.com/mmlb/yamlfmt/issues/17)

We could put this in our `make format`, and also customize it to add the `_` thousands separator.

This would also allow us to run programmatic operations over our yaml files and modify them using the yaml package, without polluting the commits with formatting changes.

Start by inspecting the existing `make format` target and how YAML files are handled with the Python YAML package. Compare the proposed `yamlfmt` and `yamlfix` tools, including whether they can preserve or add the `_` thousands separator. Done means YAML formatting is integrated into `make format` and programmatic YAML changes no longer create unrelated formatting churn.

Written by the indexing model from the issue text.