introduce directions to Ordering

[normen662]

This is a follow up to #1142 which refactored the modeling of orderings to model interesting orderings as well as plan properties. One of the things that we didn't model at all so far is the directionality of the ordering as ordering is assumed to be forward or reversed on a global level. While that is true and as we change the modeling the directionality of the ordering should be captured (and used) within the new Ordering class.