code: def unpack(self, data):
          
          size = self._get_calculated_size(self.size, data)
          self.set_value(data[0:size])
          return data[len(self):]
docs: Takes in a byte string and set's the field value based on field
          definition.
  
          :param structure: The message structure class object
          :param data: The byte string of the data to unpack
          :return: The remaining data for subsequent fields