Add Python tool for changing parameters

It's annoying to think that we're all reinventing the wheel, and it's too easy to do it wrong in hard-to-understand ways.

Ideas:
- Allow users to specify a [CSV with their desired settings](https://github.com/ESCOMP/CTSM/issues/3393#issuecomment-3168299266).
- Have that CSV use the same format as what's output by [the parameter-querying script](https://github.com/ESCOMP/CTSM/issues/3394).
- Work with both namelist parameters (save a `user_nl_clm` file) and `paramfile` parameters.
- Allow generating new `paramfile`s but _not_ changing existing ones, to avoid the possibility of changing canonical parameter files.
- Allow adding new PFTs using existing ones as a starting point.
- Facilitate use as either command-line tool or something imported into Python
- Facilitate use of _parts_ of the script in other Python scripts. E.g., provide hooks for reading and saving parameter files with our robust and tested methods, while allowing people to modify variables in their own way.