Allow H2 retrofit of gas pipelines

[lindnemi]

• It's false in config.default.yaml but should be true
• Test run with H2_Kernnetz enabled, to make sure it work together

[toniseibold]

Gas pipelines

regardless of the config["sector"]["gas_network"], the rules build_gas_network, build_gas_input_locations etc. are executed.
The results of the rules cluster_gas_network and build_gas_input_locations are all input to the prepare_sector_network rule.
There in the function add_storage_and_grids(n, costs), the following if/else statements are taken into account:
With the config of pypsa-ariadne (H2_retrofit and ~gas_network):

• gas pipelines are not added but links with a p_nom of 0 and p_nom_max of H2_retrofit_capacity_per_CH4 times the gas pipelines
• in solve_network not boundary condition is then set for the retrofit of gas to hydrogen pipelines

The prenetwork only contains links for retrofitting existing gas pipelines

n.links[n.links.carrier == "H2 pipeline retrofitted"]

and gas pipelines are not existing

n.links[n.links.carrier == "gas pipeline"]

Wasserstoff Kernnetz

Wasserstoff Kernnetz is added in modify_prenetwork.py from the file wasserstoff_kernnetz_elec_s{simpl}_{clusters}.csv
The function add_wasserstoff_kernnetz

• adds new Kernnetz connections as links
• reduces the p_nom_max of gas pipelines for the corresponding connection (not relevant for pypsa-ariadne anymore)
• reduces the p_nom_max of H2 retrofit pipelines

Bugs

#174
PyPSA/pypsa-eur#1240

[toniseibold]

With this PR in pypsa-eur the Kernnetz should now work with H2_retrofit and without a gas network
PyPSA/pypsa-eur#1258