The NVCC compiler fails to compile the date library due
to a compiler bug which causes it to emit an error when
directly using std::ratio_{multiply,divide} in the template
instantiations for std::duration. This PR works around the
issue by introducing custom ratio_{multiply,divide} which
delegate to the standard library templates
* 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.
