Treatment of threshold values in segmentation and feature detection

[JuliaKukulies]

Following our discussions in #346 and #365 we should allow users to specify if the threshold values in feature detection and segmentation are treated in an inclusive or exclusive way. We should also pick which of the currently implemented versions becomes the new default and make this default consistent between feature detection and segmentation and offer either off-ramps or an option like edge_treatment = {'inclusive', 'exclusive', 'default'}