Add foreground/background postprocessing

[wolny]

Some of the networks we have in plantseg output foreground probability maps together with boundary predictions. This can be used in the segmentation post-processing step in order to filter background instances (i.e. assign 0-label to pixels not belonging to foreground).

The easiest way to implement it as discussed with @lorenzocerrone and @qin-yu would be to add additional argument to segmentation functions in plantseg/segmentation/functional/segmentation.py called foreground_pmaps and apply filtering the output segmentation if the argument is present. Then support this parameter it in the config, legacy and napari GUI.