Script to compute urdf diff and dump it to yaml

It's a script for calibration.

1) compute diff and dump it to yaml
```sh
urdf_patch.py diff A.urdf B.urdf diff.yaml
```
2) apply the patch yaml to urdf
```sh
urdf_patch.py patch A.urdf diff.yaml B_generated.urdf
```

and B.urdf and B_generated.urdf are same.

* now only joint xyz and rpy are supported