Publish gimbal tf

[Flova]

This PR adds the ability to publish multiple ROS TF transforms.
It closes issue #1

[Flova]

This needs some testing on the gimbal setup. It should be ready for merge afterward.
Any feedback @julled ?

[julled]

you added static magic numbers for the transform for a specific gremsy gimbal, which i dont know.
maybe you could make these variables somehow gimbal specific and user accessable via configuration to also use it for various of gremsy gimbals.

[julled]

The given values seem accurate to the centimeter, is this really the case for your particular gimbal?

[Flova]

This was more or less an estimation using simple physical measurements.
Ideally, these values are loaded from an urdf file by ROS itself for the specific gimbal. Never the less the translations are not that important in most of my use cases.

Maybe I add a small example Gremsy T3V3 urdf in this PR. But not today.

[Flova]

On the other hand, we could simply use a robot state publisher for this use case. I thought it was overdone at first but now it seems like the right way. We only need to add a joint message publisher inside this node and the rest is done by ROS using the urdf. The robot_state_publisher simply matches the names provided by the joint state message to the joint names in the urdf and adds the dynamic rotations to the specified static transforms.

See: https://wiki.ros.org/robot_state_publisher

[julled]

If it is just too much work, maybe you also point out in the readme.md that the developer needs to change the values for his purpose.

[Flova]

I removed the static translational transforms from the code and they can be added in an urdf if needed.
I would test and merge this as it is and maybe add a later version using the principle from the comment above.