Pull duplicated functions into a common directory

As mentioned in this comment:
https://github.com/MetOffice/CMEW/issues/130#issuecomment-3861539221

The functions
- `write_recipe` in `CMEW/app/configure_for/bin/update_recipe_file.py` and
- `write_dict_to_yaml` in `CMEW/app/add_datasets/bin/add_datasets_to_share.py`
are actually the same function (one with a more generic name) and should be pulled into a common directory.

`CMEW/lib/python/` automatically gets added to PYTHONPATH when the workflow is run, so I initially experimented with writing `write_recipe` into `CMEW/lib/python/common.py`. The changes to unit tests (mostly the ability to run them locally) made this into a larger piece of work than anticipated, so I'm writing this as a separate issue.

Start with write_recipe in CMEW/app/configure_for/bin/update_recipe_file.py and write_dict_to_yaml in CMEW/app/add_datasets/bin/add_datasets_to_share.py, then inspect CMEW/lib/python/common.py as the proposed shared location. Review the unit-test changes needed for local execution. Done means both callers use one common implementation and the unit tests pass.

Written by the indexing model from the issue text.