Create Interface for Incumbent Selection

So far only one method is hardcoded. But other ranking methods might make sense for the incumbent selection, especially for MO.
Create a class providing the selection functionality.

Start by locating the existing hardcoded incumbent-selection method and inspect how incumbent selection is used, especially for multi-objective optimization. The issue provides no file or test names; done means selection is provided through a class interface that can accommodate alternative ranking methods.

Written by the indexing model from the issue text.