Introduce DepthOrderBook, and bind to it in SimpleOrderBook
Doxygen
Listener for BBO events.
Listener for depth events.
Add listeners, callbacks for depth update, bbo update.
Listener cleanup
Listener unit tests.
Change callback params from reference to pointer, template params to OrderBook,
Ignore ut_listeners
Comment why we save order_book pointer
Test order_book_listener
Only mark depth as changed if a visible level changed.
Use references to self's class
Properly test for depth changes.
update to assertiv with proper test failure counting
update to assertiv exiting with proper code
Test BBO callbacks.