Clean up GASP

[constantinpape]

Some potential clean-ups as follow up from #47 (I would like to keep all the public functionality functional)

• Add a functional wrapper for GaspFromAffinities
• Remove "WatershedFromAffinities"; for simple function wrappers functools.partial can be used
• Remove build_pixel_long_range_grid_graph_from_offsets in favor of functionality from elf.segmentation.features
• Remove build_lifted_graph_from_rag in favor of functionality from elf.segmentation.features
• Use elf.segmentation.compute_rag instead of get_rag (need to think about using the -1 as ignore label)

That's just what I found on first review; there may be some other things deeper in the code.

cc @abailoni