remove vestigial SingleThreadedExecutor header
add access control to groups using atomic_bool
refactor of executors for mutually exclusive cb's
added convenience debugging macro
modify factories in node to use groups

also check to make sure given groups are in node
modify rates to allow ctrl-c to interrupt sleep
added global rclcpp::sleep_for function

It differs from this_thread::sleep_for because
it will exit on SIGINT (ctrl-c).
refactor timers to use rates