Reduction of H2 retrofit pipelines when not spatially resolving gas network in myopic optimization

[toniseibold]

In add_brownfield with config["sector"]["H2_retrofit"] and not config["sector"]["gas_network"], the maximum capacity of the retrofit potential is currently not reduced. This is because the pipe_carrier = ["gas pipeline"]` which does not exist in this case.
It would be more robust to search for "H2 pipeline retrofitted" and subtract the already installed capacity from the maximum capacity.