Geoboundaries as an alternative to GADM

[virio-andreyana]

Closes #1258

Changes proposed in this Pull Request

Add the option to use a more permissive geographic administration data source. Doing this requires a few steps:

• Unified gadm() to only be from build_shapes.py. This is done by cleaning up prepare_gas_network.py. Additionally I've created the rule to make the plot for the gas network
• create geoboundaries() in build_shapes.py that is activated if the config admin_shape: "geoboundaries"
• I also made the country ISO conversion to pass through the strings that are already two or three-letter to begin with

TODO:

• (optional) rename GADM_ID and other mentions of the term gadm in administration shapes to be more neutral
• (optional) set geoboundaries as default for all functions?

@ekatef @davide-f looking forward for the technical review when you have the spare time.

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and doc/requirements.txt.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.