[PLAT-6531] Added periodic yield curve with spread
[PLAT-6531] Changes from code review, no functional changes
[PLAT-6531] Removed final from method parameters and arguments
[PLAT-6531] Added null check for curve and curveFixed constructor arguments
[PLAT-6537] Avoid use of anonymous classes, preventing fudge serialization errors in engine caching. (Chris)
[PLAT-6538] Added overrides so shock methods produce BlackVolSurface instances. Refactored VolatilitySurface to share surface building functionality. (Chris)
Merge pull request #69 from OpenGamma/topic/PLAT-6531

[PLAT-6531] Added periodic yield curve with spread