CURR11 — IDL Changes to support date ranges on ExchangeRateValue

Summary: Propose the addition of the following interface(s):

void setDateRange(in CBO::Dtime beginDateTime,
in CBO::Dtime endDateTime) – Sets the window in
time over which the ExchangeRate instance is valid.
CBO::Dtime getStartDate() – Returns the date and time at which
the ExchangeRate took effect.
CBO::Dtime getEndDate() – Returns the date and time at which the
ExchangeRate is no longer valid.

Will also need an appropriate exception to throw when the date
range is attempted to be set and beginDateTime >= endDateTime. Do
not think that the beginDateTime should be allowed to be equal to
the endDateTime as that then means that the ExchangeRate is valid
for zero seconds.

resolved by Currency 2 RTF