Make investment year and year of cost assumption independent for overnight

[ebbekyhl]

Addressing the issue #177.

The wildcard "planning_horizons" is unchanged - but input cost year (e.g., in prepare_sector_network) is now either equivalent to "planning_horizons" (for myopic) or config["costs"]["year"] (for overnight).

In that way, overnight can be run for e.g. 2050 with cost assumptions for 2030.