Consolidate cost data loading

[lindnemi]

there are two almost identical functions to load the cost data:

load_costs: https://github.com/search?q=repo%3APyPSA%2Fpypsa-eur%20load_costs&type=code
prepare_costs: https://github.com/search?q=repo%3APyPSA%2Fpypsa-eur+prepare_costs&type=code

prepare_costs does some extra preproccessing and refers to the capital_cost as fixed.
It would be better to avoid code duplication and keep only one of these.