URDF-lookup in diffdrive-controller should lookup relative positions

[MCFurry]

The diffdrive controller tries to get the wheel separation by looking up the joints in the URDF. This is a nice feature! But it assumes that the joints are directly attached to base_link (or at least a common link): source

In a lot of our cases this is not true and they each have a unique parent link and thus the lookup gives wrong results.

Wouldn't it be better to simply lookup the relative distance between the two joints instead?