PoseNet model deprecated

[federicoVisi]

Posenet model used via n4m script is now deprecated and may not work anymore: https://github.com/tensorflow/tfjs-models/tree/master/posenet

Should be replaced with one of the current ones: https://github.com/tensorflow/tfjs-models/tree/master/pose-detection

Not sure when I'll have time to do this so anyone's contribution here is welcome, just send a pull request.