Add an option to disable subworkflow

[ekatef]

Closes #35, #20 and (hopefully) #32

Changes proposed in this Pull Request

Copy-pasted changes from PyPSA-Earth-Sec intended to add an option for switching-off the sub-workflows replacing them by a direct path definitions.

Tested on mac, seems to work properly and make life much easier. @davide-f thanks a lot for the proposed solution!

Checklist

• [x] I tested my contribution locally and it seems to work fine.
• [ ] Code and workflow changes are sufficiently documented.
• [ ] Newly introduced dependencies are added to the main environment at PyPSA-Earth repository
• [ ] Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• [ ] Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• [ ] Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• [ ] A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[ekatef]

As a comment: for some reason, Snakemake linter is not happy with the changes complaining about missed keyword at the function definition for pypsaearth and usage for create_country_list. (So the commits have been possible only with --no-verify. Sorry for that!) However, both points seem to be perfectly valid approaches for Snakemake, as follows from the examples in Advanced Usage, so I'd rather say that the linter is not perfectly correct.

[davide-f]

That is weird, agree. Let's merge anyway :D

[ekatef]

That is weird, agree. Let's merge anyway :D

Thanks a lot for the idea of the fix and for merging!! Great to have the workflow running 🛫