Addition of cbrt, min, max

[pieterb@…]

LS, please consider adding the cubic root and min/max functions to the boost.units library. I have included an implementation and unit tests (all fairly trivial) in cmath.hpp and test_cmath.cpp. Maybe the min/max should be placed elsewhere as these are not part of the original cmath header-it might be nice to have a place (header) for future similar additions. Thanks for your consideration! - Pieter