Integrate scenario varying prices for commodities

[toniseibold]

For coal, oil and gas the prices are changing across scenarios. We want to integrate those into PyPSA-Ariadne as well.

First idea:

• integrate the different costs in the ariadne-data/costs_yyyy-modifications.csv files with a flag
• in rule modify_cost_data add if statements for the four scenarios

[toniseibold]

Prices do not change between scenarios for commodities