A joint planner, based on src/emc/motion/simple_tp.*

can take commands from a ringbuffer, or HAL pins

if a ringbuffer named <inst>.cmd exists, the instance will attach to it and run in queued mode
the pos-cmd pin will be OUT, as it is driven from the queue
the max-vel and max-acc pins are IO and can be overridden

otherwise it will create and honor the pos-cmd pin as IO since it is updated by the tp as well.

