Scales forecast data by the fraction of the date.
Uses the concept of decimal years to keep track of leap years. See the csep.utils.time_utils.decimal_year for details on the implementation. If datetime is before the start_date or after the end_date, we will scale the forecast by unity.
These datetime objects can be timezone aware in UTC timezone or both not time aware. This function will raise a TypeError according to the specifications of datetime module if these conditions are not met.