Improve function find_best_criteria

[casadoj]

At the moment this function (and the cross-validation version) has an attribute called min_spread of type boolean. When True, the optimization selects, among the equally good-performing, the criterion that minimizes the difference between precision and recall; when False, it selects the minimum value of the criterion.

It could be interesting to change this attribute to a new attribute named prioritize:

• min: it would select the minimum value of the criterion.
• max: it would select the maximum value of the criterion.
• min_spread: it would select the minimum difference between precision and recall.