Improve metric subclasses organization

[csala]

The current class organization includes some metrics which are usable on their own, like BNLikelhood, which have subclasses and therefore are not picked by the get_subclasses method. As a result of this, BNLikelihood is not available in the sdv.evaluation.evaluate function: https://github.com/sdv-dev/SDV/issues/327

The class organization and subclass selection should be reviewed to ensure that all the usable metrics are properly selected by the get_subclasses method.