Requirements on parallel_reduce body

Class Body implementing the concept of parallel_reduce body must define:

• Body::Body( Body&, split ); 

  Splitting constructor. Must be able to run concurrently with operator() and method join

• Body::~Body(); 

  Destructor

• void Body::operator()( Range& r ); 

  Function call operator applying body to range r and accumulating the result

• void Body::join( Body& b ); 

  Join results. The result in b should be merged into the result of this