omCosts should be p_gnu and not p_unit

omCosts are currently summed for all output v_gens. This can be wrong in some circumstances. An update should move omCosts to p_gnu and change the equations accordingly. Need to think whether should potentially apply to inputs as well.

Furthermore, it should be considered whether all costs could be added into 'productionCost' before sending the problem to the solver. This is not always possible (flexible switching between two outputs), but could reduce computation time in some cases. First: check if there is a computational gain. If so: make a new issue for the purpose.