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
`Frequency` has a special concept of `TERM` which effectively models a zero coupon. `Tenor` is a wrapper defining the length of the trade, and as such is always positive.