Connection mode transfer protocol J1939-21

[sheckey]

The send_pgn() argument list does not appear to support sending a connection mode transfer to a destination address other than what is also specified for the PGN to be transferred. For example, in this request to send message, the destination address is 0x1C and the PGN of the transfer is 0xFF73:

r xtd 02 18EC1C03 08 10 70 00 10 10 73 FF 00 0 0 10.533150 0.114073 .p...s.. TP.CM RTS - JD Product Identification

With the library as it is, we cannot specify 0x1C separately from 0xFF73 , but only PDU format and specific which goes in both places: the CAN ID and the message body. I have hacked an easy solution, but would like to discuss this to see if I am incorrect, or using it incorrectly, or if you would like to me to try and make a pull request to help out.

Thank you for making this wonderful library available!