Distinct only works on single fields

[torpedro]

Distinct operator can only handle a single definition of field, but we specify a list of fields in the json.
https://github.com/hyrise/hyrise/blob/master/src/lib/access/Distinct.cpp#L27

In the documentation it sounds like it could handle multiple fields.
https://github.com/hyrise/hyrise/blob/master/docs/documentation/queryexecution/json_plan_operations.rst

Plus: The distinct operator returns all columns, not only those that have been specified in the distinct. This is not the behaviour that one would expect when doing a SELECT DISTINCT in SQL. I think it should be the same here.