Clean up the mess of when models, info files, engines etc are saved

[johann-petrak]

This got messy when we had to replace the proper weka models with the external approach. Now we do not save the info file if the model is null and leave that to the algorithm-specific saveModel method. Since LibSVM does not have its own model class, the info file is now saved in libsvm's save model method. We need a clean way of how to do this, as easy as possible for a user of the API.