Allow flexible post-processing for larger segmentations

[anwai98]

I realized that there are some further post-processing steps (two to be precise), which are tricky for larger segmentations:

• There's one in generate method for automatic segmentation, where it does relabelling and some other stuff. This can be overridden by setting output_mode to None (passed to the generate method) via the automatic segmentation functionality.
• There is another one after the segmentations are returned and some filtering based on area per object is done (intended for AMG mostly).

This PR takes care of making the second point flexible. GTG from my side!