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`

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