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.
Do we need an isExactlyDivisible() method to allow you to check whether you can make the exact divide call? (cf Iterator hasNext() and next())