[PLAT-5350] Adding an abstract base class for curve specifications
[PLAT-5351] Adding a constant curve specification
[PLAT-5349] Adding a spread curve specification
[PLAT-5350] CurveSpecification now extend AbstractCurveSpecification
[PLAT-5351], [PLAT-5349] Adding and testing fudge builders for constant and spread curve specifications
[PLAT-5351] Constant curves can now be created and used for bond analytics
[PLAT-5366] Adding support for spread curves