Update / synchronize _helper script and dependencies with main project

[fhg-isi]

Compared to pypsa_eur, this project uses an outdated version of _helpers.py
from snakemake.settings import

vs

from snakemake.settings.types import

=> Try to avoid copy paste code for _helpers module.
=> Maybe mocking snakemake would not be required if using run instead of script in snakemake file and reworking the interface between snakefile and script. Then _helpers.py could be removed.
=>If helpers.py is still needed, maybe move common code to extra project to reuse functionality instead of duplicating it.
=> Update dependencies.