get_payload
Description:
Get the MIKEYPayload at idx in 
      this
Parameters:
| this | |
| idx | an index | 
Returns:
| the MIKEYPayload at  | 
Get the MIKEYPayload at idx in 
      this
| this | |
| idx | an index | 
| the MIKEYPayload at  |