[boost] Numeric Conversions Library added to CVS

Hi All, In preparation for the upcoming 1.32 release I added this library to CVS. The code and docs have been changed according to the extensive review results. I still need to extend the documentation adding examples and motivating cases though, which I plan to do before the release. I haven't yet changed "boost/cast.hpp" to use the new stuff, I will as soon as I see the code stablilized among different compilers. In the meantime, new users can use "boost/numeric/conversion/cast.hpp" which contains the new version of numeric_cast<> Best Fernando Cacciola SciSoft