New stream sub features, TF tree search Service

[leshy]

getter = stream.hot_latest() # keeps the sub active, returns immediately latest value
getter() -> Odometry
getter() -> Odometry
getter.dispose() # unsub

stream.get_next() -> Odometry # subs, waits for next, returns

stream.observable() # whatever you want

also fixes & documents TF system

[leshy]

#504 will be merged with this