usermod for generating crop calendar inputs

I need to update the [documentation for generating crop maturity requirements](https://escomp.github.io/CTSM/users_guide/running-special-cases/Running-with-custom-crop-calendars.html#generating-maturity-requirements) (#3332), and also add instructions for generating baseline GDD20 files (#3663). Those processes use outputs from the same run. I think it would be a good idea to make a usermod that sets up such a run, to avoid having to update the documentation for minor changes in the future.