Requirements on parallel_scan body

Class Body implementing the concept of parallel_scan body must define:

• Body::Body( Body&, split ); 

  Splitting constructor. Split b so that this and b can accumulate separately

• Body::~Body(); 

  Destructor

• void Body::operator()( const Range& r, pre_scan_tag ); 

  Preprocess iterations for range r

• void Body::operator()( const Range& r, final_scan_tag ); 

  Do final processing for iterations of range r

• void Body::reverse_join( Body& a ); 

  Merge preprocessing state of a into this, where a was created earlier from b by b's splitting constructor