radard: calculate lead acceleration on cluster of tracks

or initialize from the model (it's a prediction on two frames rather than three, so it cannot ever be too accurate currently).

Right now we track lead accel per reported radar track. Lead accel is very important for the planner during high lead accel/decel situations.

We are fortunate that Toyota's reported radar tracks are mostly self consistent, in that a position/lead in space doesn't switch very often between the messages. However, it does happen. Here we could have reached 1 m/s^2 20% faster if the lead accel did not oscillate/start from 0:

https://connect.comma.ai/a2bddce0b6747e10/0000030f--995be3b11f/153/182