Make the market zone clustering more flexible

[ulfmueller]

The aggregation in the build_market_model() to market zones i.e. per country and DE and LU combined is very hard-coded. If one would change the market zones e.g. devide the German one into two, it would be nice to have a more flexible functionality.

[ClaraBuettner]

A parameter for market_zones was added to the args in 116ee82 . "status_quo" is currently the only option but if we want to add new market_zone options, we could make use of this.