Develop H2 grid clustering

[ulfmueller]

As our data models will have a H2 grid very soonish, I think we need to adjust the H2 grid clustering. So far the H2 buses are automatically clustered depending on the number of CH4 buses (see here). This could even stay like it (although we might want to have different ch4 to h2 grid resolutions (another question!)) - but what we definitely need is some code which aggregates the new H2 lines. As I see it the gas_postprocessing function has to be also executed and slightly (hopefully) adjusted for the h2 grid and a corresponding busmap.
Who should tackle this? @CarlosEpia @lenzim97
@CarlosEpia you probably have the most knowledge here, right?