Add spec for PremiumSchedule#drawdowns
Move drawdowns_taken_by_quarter logic to PremiumSchedule
Refactor core logic to new OutstandingDrawdownValue class
Refactoring of premium payments calculation

Replace premium_calculation_strategy column with legacy_premium_calculation
(boolean).

Refactor logic out of *PremiumPaymentCollection and PremiumPayment classes
into PremiumSchedule and OutstandingDrawdownValue.
Remove unused *PremiumPaymentCollection and Premium Payment classes
Use accessor method