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