requirements.txt lockfile for accessible reproducible environements

[pgarrison]

Purpose

The current README instructions ignore the pdm.lock file and install from pyproject.toml, which does not create reproducible environments. We may not expect all users to have pdm, though.

Changes

• Export the pdm.lock to requirements.txt format, and update the instructions to use it.
• Add github actions to enforce that the requirements.txt is up to date with the pdm.lock and that the pdm.lock is up to date with the pyproject.toml.

Testing

• I did the steps python -m venv venv, source venv/bin/activate, and pip install -r requirements.txt, but didn't run any code in that new environment.
• The github actions failed (as expected) before updating pdm.lock and now succeed