Warnings

In the presentation we mute the warning messages, which is good for some, for some other we should discuss why they are produced, notably: 

```
4: In checkConv(attr(opt, "derivs"), opt$par, ctrl = control$checkConv,  :
  Model failed to converge with max|grad| = 0.00464817 (tol = 0.002, component 1)
```

Cause the model fails to converge! See also #5