Endogenous steel and endogenous hvc config

[toniseibold]

When setting config endogenous_steel: true and endogenous_hvc: true, the loads appear as NaN.
This is due to add_industry in prepare_sector_network.
Somehow the variable p_set is not grouped for each node which can be added as
p_set = p_set.groupby(level="node").sum()

For further investigation it would be crucial to know whether this is a pypsa-eur or pypsa-ariadne problem.