partial_split_message
Description:
Splits this into an array of Message objects each 
      containing a single MessagePartial object containing max_size 
      bytes or fewer.
 nparts is set to the number of MessagePartial objects created.
    
Parameters:
| this | message object | 
| max_size | max size | 
| nparts | number of parts | 
Returns:
| an array of Message objects and sets  |