topic/PLAT-6783b

[PLAT-6779] Simple utility to calculate the number of business days between 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