Store network shapes

[FabianHofmann]

In addition to the .geojson-files that are stored in the resources-directory, the shape files are stored inside the pypsa.Network-object and consequently stored as .nc-files.

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.