docs: def normalize_volume(volume):
          '''convert volume metadata from es to archivant format
  
             This function makes side effect on input volume
  
             output example::
  
              {
                  'id': 'AU0paPZOMZchuDv1iDv8',
                  'type': 'volume',
                  'metadata': {'_language': 'en',
                              'key1': 'value1',
                              'key2': 'value2',
                              'key3': 'value3'},
                  'attachments': [{'id': 'a910e1kjdo2d192d1dko1p2kd1209d',
                                  'type' : 'attachment',
                                  'url': 'fsdb:///624bffa8a6f90813b7982d0e5b4c1475ebec40e3',
                                  'metadata': {'download_count': 0,
                                              'mime': 'application/json',
                                              'name': 'tmp9fyat_',
                                              'notes': 'this file is awsome',
                                              'sha1': '624bffa8a6f90813b7982d0e5b4c1475ebec40e3',
                                              'size': 10}
                              }]
              }
  
          '''
          res = dict()
          res['type'] = 'volume'
          res['id'] = volume['_id']
          if '_score' in volume:
              res['score'] = volume['_score']
  
          source = volume['_source']
          attachments = source['_attachments']
          del(source['_attachments'])
          del(source['_text_' + source['_language']])
          res['metadata'] = source
  
          atts = list()
          for attachment in attachments:
              atts.append(Archivant.normalize_attachment(attachment))
          res['attachments'] = atts
  
          return res