Make copying default config file compatible with snakemake module

[koen-vg]

It appears that the code for copying the default config to config.yaml wasn't compatible with using pypsa-eur as a snakemake module. When that is the case, it's important to use paths relative to workflow.current_basedir throughout (though confusingly current_basedir is not available in included snakefiles, where the next best thing is workflow.snakefile).

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.