This methods takes a trainer and a training set and returns the performance
(in [0,1]) of the trained classifier on the training set (e.g. using
cross-validation). The larger the value the better the trained classifier.
Measures the performance of the given classifier on the given test set, i.e.
every observation from the training set is classified by the classifier and
its prediction is compared with the provided category. The return value is the
ratio of the correctly classified observations and the total number of