Idyntree: Remove direct indeces uses from existing examples

[traversaro]

The example https://github.com/robotology/idyntree/blob/master/examples/matlab/iDynTreeWrappers/visualizeRobot.m#L93 directly uses indices to refer to joints and links. This should always be avoided, as indices can change if the model changes, and so any use of indeces should only come from the model.getLinkIndex or model.getJointIndex methods, to ensure that it works fine.

[traversaro]

If any existing user of the MATLAB visualizer could handle this it would be great. @nunoguedelha @CarlottaSartore @Giulero @singhbal-baljinder @gabrielenava @lrapetti

[traversaro]

Fixed by @lrapetti in https://github.com/robotology/idyntree/pull/810 .