PLT-513 Add Frequency.exactDivide()

Provides a mechanism to divide one frequency by another

When working with schedules, it is useful to be able to divide one frequency by another. This operation is only expected to succeed if the frequencies are compatible, such as dividing a payment frequency of P6M by an accrual frequency of P3M.
Could do, but currently no need for it, and similar `eventsPerYear()` method doesn't have an `is` method either.