Feature market_simulation follow-up CH4

[ulfmueller]

This branch introduces a first working version of a consecutive market and grid optimization. Here is one follow-up todo: The gas sector clustering is not explicitly addressed inside of the market simulation. So the resolution stays as it is given by the parameter in the overall args for the grid model. So, it would be good to introduce s.th. inside of the market simulation going always to CH4 nodes (one per country), I suppose. And then the grid model CH4 complexity is independent from that (if all relevant components are not aggregated and do not need any disaggregation).

[ulfmueller]

Closed in favor of a dublicate newer issue #700