Add Kinova Link6 model

[anashoussaini]

If you would like to see a new model added to the Menagerie collection, please provide us with the following information before sending a pull request. (Ideally, before you start working on converting or tuning the model for MuJoCo.)

URL for the existing model source (e.g. URDF)

• We have the URDF internally only.

License terms of the upstream model
Modified BSD License.

Are you proposing to help develop and maintain the new model?
Yes, I work at Kinova, we will be maintaining this repo.

[kevinzakka]

Hi @anashoussaini, same for this one, a PR would be appreciated :)