deal with stores in foreign countries

[ulfmueller]

to be consistent with the inner-German expansion assumption, not just the links from p-e for the foreign countries like it is already done in the data model (see here) have to be extendable. As some of the stores like batteries have get e_nom due to expansions from the scenario path their e_nom_extendable is false. I think we simply have to set them to True, since the e_nom_min is 0 the e_nom is then neglected and we can use it as an option or for comparison.
Coming from the links I would tackle these store carriers from p-e (they have slight different names in our data model):

       ["battery",
        "home battery",
        "rural water tanks",
        "urban central water tanks",
        "urban decentral water tanks",
        "H2 Store"]

[ulfmueller]

it is also linked to #777 since batteries wont be stores once that issue is solved.