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
I don't think this is what we want to do. If a user enters 27M, then they would expect to see it as 27M, if they enter 2Y3M they should see 2Y3M. That is not to say that we shouldn't normalise the periods, but the name should reflect what the user has entered. I checked with @anujgup and this was his view too.