Use s_nom_min for the reference point for the transmission
reinforcement not s_nom, since s_nom is now overwritten for LV > 1.0
(the old pyomo code kept the s_nom variable fixed in the pyomo model
rather than via pypsa setting s_nom_extendable = False, like the
nomopyomo code now does).
The legend scale of the H2 network electrolyzers is wrong. I don't yet
understand why. Possibly the cost scale in the other map is also wrong.
With new pandas:
pd.Index([])|pd.MultiIndex(...)
returns a pd.Index, not a pd.MultiIndex, so just reversed:
pd.MultiIndex(...)|pd.Index([])
This returns a pd.MultiIndex.
