Requirements on parallel_scan body

Class Body implementing the concept of parallel_reduce 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

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