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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