Requirements on parallel_do body

Class Body implementing the concept of parallel_do body must define:

        B::operator()( 
                cv_item_type item,
                parallel_do_feeder<item_type>& feeder
        ) const
        
        OR

        B::operator()( cv_item_type& item ) const

Process item. May be invoked concurrently for the same this but different item.

```
 item_type( const item_type& ) 
```
Copy a work item.
```
 ~item_type() 
```
Destroy a work item

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