Move from subworkflows to modules in snakamake to ease usage on windows

[davide-f]

Checklist

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

Describe the Bug

Please provide a description of what the bug is and add a minimal example/command for reproducing the bug.

Error Message

If applicable, paste any terminal output to help illustrating your problem.
In some cases it may also be useful to share your list of installed packages: conda list.

<paste here>