added calendar-arithmetic tasks

[bjiro]

Added 7 tasks for calendar arithmetic.

For prompt diversity, each has 3 question templates and they share different format requests selected at random (eg: specify if the expected answer is a day in letters or a numerical value).

The calendar arithmetic problems (generally) span a year, specified in the config. The tasks have different inherent complexities, with three parameters that can be tuned to adjust the difficulty:

Tunable:
"is_leap_year" - tuned with leap_year_range

The next four tasks difficulty depend on offset_upper_bound. They can be easy to hard, by setting the offset bound from a few days to an arbitrary large number.
"weekday_offset"
"count_business_days"
"count_days"
"weekday_of_date_from_first_day" (offset is capped to fit in a year)

Not tunable (and hard, especially if given year is not in the training distribution of the llm):
"weekday_of_date"
"recurring_event_day" (harder)

I also created a denser reward for numerical answers and for strings (eg: wrong capitalization shouldn't have 0 reward).

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[andreaskoepf]

Great contribution! Thank you very much!