Should "sort" and "indexScan" be implemented in the appropriate Operations?

[joka921]

Currently the actual implementation of Sorting an (intermediate) result is provided by a class called Engine and only used by the Sort operation. Similarly, the functionality of an index Scan resides in the Index class and not the IndexScan class. We should thoroughly check, if there are good reasons for this architecture or whether we can move this functionality to the appropriate Operation subclasses.