variable costs pricing

[stefansc1]

Add pricing strategy variable_costs. Not a charging strategy, but takes procurement and commodity costs from given price_list instead of fixed from price_sheet.
Also some minor refactoring in costs.py: remove backslashes, more lenient PV warning.

[j-brendel]

Right now the assumtion seems to be that "procurement" is always given in "price_list".

However, the assumtion should be that either one of "procurement" or "commodity" or both exist.
In none of both columns/dict entrys exist, the cost_calculation should abort with an error "variable price method chosen, but neither variable procurement nor commodity prices given".
If one of both does not exist, the valid parameter should be taken from the price_sheet/ cost_params in the same way as in the "fixed" cost strategies.

[j-brendel]

beautiful!