[PLAT-6355] : add tests for jacobian matrix in MulticurveBuildingDiscountingDiscountUSDSpreadTest.
[PLAT-6356] : add function getCurve and getFixedCurve in YieldAndDiscountAddZeroFixedCurve. (Marc)
[PLAT-6357] : add function getCurve and getFixedCurve in YieldAndDiscountAddZeroSpreadCurve.(Marc)