Refactor the naive_fusion functions by show_overlaps and respect_probs

[gatoniel]

The functions are too complex as the flake8 complexity was now raised to 17. This is due to the constant if statements for show_overlaps and respect_probs. The functions should be separated in different functions so that the if statements are in the very beginning and not within each step of the while-loop.

[gatoniel]

Also add numba @jit in order to speed up stuff.

[gatoniel]

Another speed up could be reached by rewriting the polygon_to_label and polyhedron_to_label functions from stardist without there constant type checking.