extract_results should use percentages for accuracy, since vipercharts widgets appear to expect them

added widgets that enable mapping unseen _relational_ examples to a precalculated _propositional_ domain for RSD, TreeLiker and Aleph-featurize

orange discretization widget now also accepts a dictionary of pre-defined discretization values for each table and attribute

fixed a bug where databases with loops would crash treeliker, since an incorrect conversion would be made