Implement alpha costs that are applied only during commission periods

-) Add commissionPeriod parameter
-) Add CommissionPeriod cost duration dimension (i.e. add one value to the corresponding dimension of the costs array)
-) Add alphaCostsCommission parameter to indicate how alpha costs over the commission period are to be understood: actual / linear / presentvalue

-) Add adjustPresentValues(Costs) hook functions to allow modifying present values

-) Add benefitParameter parameter to pass contract-/tariff-specific information to custom functions