Add variation of drug cycling strategy that is synchronized with the calendar

[rjzupkoii]

While documenting the the CyclingStrategy class it was observed there is a comment that "cycling_time should be match with calendar day." As implemented the class is strictly based upon elapsed days which can result it moving out of sync with the calendar as a result of leap years. Updating the class to support alternative calendar spans (i.e., months or years) would allow the system to stay synchronized. Alternatively, a leap-year aware version could be added to the codebase.