Redundancy in Sort and OrderBY Operation

[joka921]

We currently have two subclasses of the Operation class which both handle the sorting of an input, called Sort and OrderBy.
One of them is used for an ordering that was specified by the user via an ORDER BY clause in the query, and the other is used for orderings that are automatically inserted for technical reasons (e.g. because some operations require a sorted input).

Since both classes provide the exact same functionality, there is a lot of code duplication here and we should get rid of one of them for better maintainability.

[joka921]

This is not up to date anymore as we will need two operations with different Semantics: "Sort" is sorting on internal IDs for joining whereas "ORDER BY" is the sorting according to the actual values of the IDs such that end users get the desired order.