[accumulator] Generalise type arithmetics, e.g. make it play well with boost.units.

[alfredo.correa@…]

Accumulators library assumes that for an accumulated type T*T is also of type T. This makes it difficult to play it well with boost.unit for example; specially for the variance accumulator.

Based on ​http://groups.google.com/group/boostusers/msg/86ebe100e3a86794 Below is the goal code:

#include<boost/accumulators/accumulators.hpp> #include <boost/accumulators/statistics/variance.hpp> #include<boost/units/systems/si.hpp> #include<boost/units/io.hpp>

using namespace boost::accumulators; using namespace boost::units; int main(){

accumulator_set<

quantity<si::time>, features<

tag::variance,

a;

a(1.0*si::second); a(2.0*si::second); a(3.0*si::second); std::cout

<< extract::variance(a)

; return 0;

}