Integrate pypsa-eur as a git submodule

[fneum]

Instead of placing pypsa-eur in a parallel folder, it might make sense to directly integrate it with this repository via git submodules (https://github.blog/2016-02-01-working-with-submodules/) which align very well with snakemakes sub-workflows. This would simplify the installation and make it more reliable.

git submodule add https://github.com/pypsa/pypsa-eur 

Update and initializizing the submodules (also recursively e.g. submodules of submodules) (similar to git pull) can be done with

git submodule update --init --recursive

Something to keep in mind once this repository goes public.

[fneum]

Could also look at snakmake internal modularisation of external workflows (new in 6.0)

https://snakemake.readthedocs.io/en/stable/snakefiles/modularization.html

esp. https://snakemake.readthedocs.io/en/stable/snakefiles/modularization.html#modules can handle git repositories.