Work around for a NVCC compiler bug

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

test/date_test/multi_year_duration_addition.pass.cpp

@@ -72,8 +72,8 @@ main()
     using namespace date;
     using namespace std::chrono;
 
-    using decades = duration<int, std::ratio_multiply<std::ratio<10>, years::period>>;
-    using decamonths = duration<int, std::ratio_multiply<std::ratio<10>, months::period>>;
+    using decades = duration<int, date::detail::ratio_multiply<std::ratio<10>, years::period>>;
+    using decamonths = duration<int, date::detail::ratio_multiply<std::ratio<10>, months::period>>;
 
     constexpr months one_month{1};
     constexpr years one_year{1};