Minor changes and refactoring for psi_pred and psi_truth

Minor changes and refactoring in:
    * structured output machines
    * structured output models
    * CResults structure
to remove result->psi_pred and result->psi_truth
Seems that you're patch introduces additional overhead: It's computing joint vector twice: One time inside Model::argmax and one time in machine.

I think the slack-computation should now be done inside the machine, not the model.  As this variable is only printed and doesn't contribute to the result, this shouldn't hurt.