[PLAT-6413] AnnuityAccrualDatesVisitor does not support definitions of type CouponONSpreadDefinition

Adding visitor
[PLAT-6413] AnnuityAccrualDatesVisitor does not support definitions of type CouponONSpreadDefinition
Merge pull request #25 from OpenGamma/topic/PLAT-6413

[PLAT-6413] AnnuityAccrualDatesVisitor does not support definitions of type CouponONSpreadDefinition