Apply etrago functions on market model

[ClaraBuettner]

Check which functions should be available for the market model and allow applying them to that network.

• [x] csv export
• [x] csv import
• [x] load shedding
• [x] country tag
• [x] plotting

calc_results could also include sth. new, but it seems to be nice-to-have so I excluded this so far