PLT-466 - Refine definition of Frequency and Tenor

Use months when less than 24 months and years above that
Unify definition of 1 year to be 12 months
Unify definition of weeks vs days
Extra tests
We have used a zero period in the past as a special flag indicating a zero coupon swap (i.e. there is one payment at the end of the swap) - trade inputs seem to define this as either zero coupon or 1T (where T implies term of the trade i.e. 1 payment at the end of the trade).