Provide interoperability between std::chrono::duration and boost::units::quantity<time>

The time dimension is special in C++, because there already is an implementation in the standard libraries: `std::chrono::duration`.
I think boost::units should recognize this and provide an easy way to convert between these two different implementations of what is basically the same thing.
Ideally one should be able to do:
`quantity q = 2h;`
where 2h is a `std::literals::chrono_literals::operator""h`.