Updated to new APIs

[AlessioLovato]

DO NOT CLOSE

Pull request to update the example script at the new API version.

The example ipynb for JSON format has been updated, but the YAML version has still some issues (add_socket function is not available anymore)

[EdoardoRomiti]

Hi @AlessioLovato thanks for working on this. A suggestion regarding the new API: functions like add_socket are not available anymore, but the standard add_module() can be used.
For example:

urdf_writer.add_module('socket.yaml', offsets={'x': 0.0, 'y': 0.0, 'z': 0.0, 'roll': 0.0, 'pitch': 0.0, 'yaw': 0.0})

P.S. the kwarg offsets is optional

[AlessioLovato]

Aims to solve #12