Surface velocity when Track velocity is zero

[stefaniapedrazzi]

It seems that even if the Track velocity is zero, the surface velocity between bodies is set (to 0): https://github.com/cyberbotics/webots/blob/master/src/webots/engine/WbSimulationCluster.cpp#L384

As a result, a robot that could climb a diagonal surface is not able to climb a diagonal track with the same orientation when the track velocity is zero.

We should evaluate if it is not better to check if the Track velocity is different from zero before setting the surface velocity.

Or check if there is a better way to add the surface velocity generated by the Track to the normal surface velocity generated by motors wheels, etc.