Topic/reqs 648

* Implemented **SwapCleanDiscountingCalculator** and its test **SwapCleanDiscountingCalculatorTest**. 
The comparison should be done on LocalDate and not on ZonedDateTime, to avoid jumps during the day. 
Also, according to the doc, the payment on the valuation time should not be included. The condition should be paymentDate.toLocalDate.isAfter(valuationDate.toLocalDate).