Misapplication of ancient snakemake keyword

[koen-vg]

It looks like the recent commit marking the databundle as ancient instead of protected isn't really working; ancient is only supposed to be used for inputs, not outputs. Somehow it looks like it's actually making the whole workflow re-run every time for me.

I think maybe it would be an idea to write a little piece of Python code in retrieve.smk to check if all the databundle output files exist, and only define the retrieval rule if they don't. That should make it so snakemake really doesn't try to re-run the data bundle retrieval unless file are added or you manually delete it.

I do get the motivation because whenever I rebase anything, snakemake usually says that the code changed and it want to re-run the data retrieval.

Checklist

• I am using the current master branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-eur environment. Update via conda env update -f envs/environment.yaml.

[koen-vg]

I'll see if I get around to a PR today.