* Added new overloads for operator+/- between year_month and duration
that is convertible to years, so it is better candidate for operands
that are convertible to both years and months. To preserve
functionality, this operator is conditionally noexcept.
* Reworked year_month_day, year_month_day_last, year_month_weekday,
and year_month_weekday_last.
* Added tests for this new functionality.