Tidying up in SBP code.
Started SBP unit tests.
Adding checks for null arguments to sbp_register_callback.
Add function to clear all the SBP callbacks currently registered.
SBP unit tests now test the callback registry quite thoroughly.
SBP - add unit tests for registering duplicated callbacks.
SBP unit tests - add fixture to clear callbacks before each test.
SBP - add tests for null arguments to send_message.
SBP - handle case where len = 0 better in send_message.
SBP - starting unit tests for send_message
SBP - unit tests for send_message encoding against hand calculated examples.
SBP unit tests - for portability don't use GCC attributes.
SBP - started tests for sbp_process
SBP - fix bug where state was not reset if there was an error.

(yay for unit testing)
SBP - more unit tests for sbp_process.
SBP - one more test for sbp_process.
SBP - clean up