add low-level damp mode

[Glonks]

Adds a low-level damping mode. Transitions to emergency mode are now more simple.

Transitions now look like this:

• High -> Emergency: Simply call damp_high() (via LocoClient::Damp())
• Low -> Emergency: Our custom damp_low(). Sends a 0 position, 0 velocity, 0 effort command with 0 kp and Damp::kd kd values
• Idle -> Emergency: Determine whether the internal high-level controller is active and use either damp_high() or damp_low()