Make CO2 intensity limit compatible with hydrogen load

[nworbmot]

Currently you can apply a CO2 intensity limit in tCO2/kWh. However, this is only applied to the electricity load, by multiplying the electricity load with the CO2 intensity to get a yearly CO2 budget. If you also have a hydrogen load, then this is not added to the budget. This should probably switch to an overall electricity generation constraint.