returns the date at which the event occurs
returns true if an event has already occurred before a date
										overloads Event::hasOccurred in order to take
									Settings.includeTodaysCashflows in account
This method should be called at the end of non-const methods or when the programmer desires to notify any changes.
returns true if the cashflow is trading ex-coupon on the refDate
Base class for cash flows
This class is purely virtual and acts as a base class for the actual cash flow implementations.