Publishing odometry for 6w_Drive plugin

[l0g1x]

Uncommenting publish_odometry() function. Not sure if this was originally done on purpose or not.

[flabrosse]

Since 2014, the call to publish_odometry() is still commented out. Is there a reason for this? The odometry is calculated, why not publish it?

[qhdwight]

Any chance we can get this merged in? Would be super helpful. Although I'm not sure if the tf logic is correct, seems to not agree with the odometry publishing.

[Martin-Oehler]

There are several problems with this code. The transform is directly published to tf instead of a separate topic. The tf frames are hardcoded. And most importantly, this just publishes the ground truth instead of using diff drive kinematics as one would maybe expect. Therefore, in its current form, this won't be merged.

If you are interested in the ground truth transform, you can use GazeboRosP3D.