Add the possibility to load the urdf model at runtime

[GiulioRomualdi]

Thanks to robotology/icub-models#130 it is now possible to access the location of the urdf model directly from python. Consequentially it would be nice to implement the possibility to switch the robot model in the visualizer (this will allow using the visualizer also for the other research axis). The tasks required are:

1. read the joints list from the mat file. (the list is already there so we should just read it from python)
2. implement a function that removes an existing model from the visualizer (meshcat)
3. add a QComboBox to select the model
   

If there is someone willing to implement this feature, I can guide them.

cc @ami-iit/artificial-mechanical-intelligence

[GiulioRomualdi]

The name of the model is retrieved from the mat file a985a69