I'm not sure to see the relation of velocity, for me, it looks like really low-level odometry sensor.

add_propery() lines are too long. wheel_base and wheel_radius should probably be robot property, not sensor property. It may be nice also to be able to extend to caterpillars

Also, it would be nice to be able to test at builder time that the sensor is configured properly for an 'differential robot', not a 'standard one'.

@adegroote Indeed, Odometry is a better place (I started there, then I switched to Velocity... I do not remember why...). Updated accordingly the PR.
While here, added a unit-test (do not pass on my computer :-( precision issue? rounding error? feedback welcome).

add_property lines wrapped.

For the wheel_radius and wheel_base, I agree that those should be managed at the robot level. This would likely require first the creation of the DifferentialDriveRobot parent class. I can create a separate issue for this if you wish.

The comment speaks about dtheta, while the code use dw, it is a bit confusing.

Considering the unit-test, I think there is several issues. The precision is probably not really good per se, as you basically add a lots of small float. On the other side, the unit-test use a fixed DELTA, while the error definitively increase at each "step". If you want to keep a fixed DELTA, you should probably store last_{left, right}_encoder and compare with them.

Concerning DifferentialDriveRobot, PhysicalWheelRobot contains most of the needed logic (if not all).