topic/PLAT-6783b

[PLAT-6779] Simple utility to calculate the number of business days betw..een two dates
[PLAT-6783] The method present value of VarianceSwapStaticReplication has a fencepost error
[PLAT-6785] Remove observation frequency from VarianceSwapDefinition and EquityVarianceSwapDefinition
[PLAT-6781] toDerivative method of EquityVarianceSwapDefinition and VarianceSwapDefinition has bugs
deriv.getTimeToObsStart() == 0.0 case can be included in this branch?