Investment cost of underground natural gas storage too low by a factor of ~7

[koen-vg]

Checklist

• I am using the current master branch or the latest release. Please indicate:

Describe the Bug

The gas storage investment cost uses the wrong data field. We set:

def add_gas_storage(data):
    # [...]
    investment = gas_storage.loc['Total cost, 100 mio Nm3 active volume'].iloc[0]

Unfortunately, as far as I can tell, the "Total cost, 100 mio Nm3 active volume" field simply tells us the cost of the natural gas needed to fill the the 100mio storage cavern. Actually using the "Specific investment [MEUR/MWh]" field gives a value that is about 7 times higher.

The documentation for the add_gas_storage function helpfully tells me that "add gas storage tech data, different methodolgy than other sheets and therefore added later". Could someone chip on why the methodology is different for gas storage? To me it looks like we clearly need to drop the manual investment cost part of that function. Maybe the charger/discharger parts of the function should remain, but gas storage itself could be added just like e.g. hydrogen underground storage is added (no special treatment needed)?