Parallelizing superpixel segmentation

[abs711]

A parallelized implementation of: https://github.com/DigitalSlideArchive/HistomicsTK/blob/7a394230dbc676c1d60664478f8f8be09f61ddda/histomicstk/cli/SuperpixelSegmentation/SuperpixelSegmentation.py#L17C5-L17C22