Add functionality to clean workflow outputs

[ekatef]

Thank you for the great package :)

Describe the feature you'd like to see

When playing with the model, it feels like it may be a good idea to have an opportunity to quickly clean-up all the workflow output to prepare the model to the next run.

Implementation may be as easy as snakemake -j 1 solve_all_networks --delete-all-output added to README, or adding a clean rule like it's done in pypsa-earth.

Happy to open a PR on that, if you think it could be useful :)

[TomFer97]

Completely agree