add combined sender+receiver class with single socket

[ottojo]

Hi! I find the socketcan wrapper classes very useful, the design of seperate receiver and sender however has one limitation: Since each has its own socket, it becomes difficult to ignore messages sent by the same ros node. #45 disables loopback by default, but that stops the use-case of running the same node twice and have it communicate (for testing purposes) via a (v)can interface.

Attached is an implementation to try it out, and it works as expected: The transceiver does not receive messages it has sent, but another instance of the same ros node receives its messages.

I wanted to ask if something like this would be considered for inclusion, before looking at implementing it properly (it's just the bare minimum sender and receiver copy-pasted together right now). I would imagine at least moving some implementation details to socket_can_common, or maybe it would be possible to just derive the Transceiver class from Sender and Receiver.

See also the linux kernel docs for CAN_RAW_LOOPBACK and CAN_RAW_RECV_OWN_MSGS.