[boost] [numeric_cast] request for boost::numeric_downcast

Hi Fernando, Amoung our utilities we have boost::polymorphic_cast boost::polymorphic_downcast The latter only performs a check in debug builds, and can then be used whenever the cast must succeed. boost::numeric_cast has no such alternative. Therefore I wuold like to see one. To keep in the naming convention from above, I think it could be reasonable to call this boost::numeric_downcast Furthermore, this should if possible be implemented in a seperate header (which is included in the main header) that can be compiled without exception-support. I think there are still platforms where they need this functionality, but don't want to sue exceptions. Any comments -Thorsten