sparse_quantize with return_index = True

[jac99]

I'm not sure if this is by design, but currently when sparse_quantize is called with return_index set to True it returns only unique_map (indexes of selected elements). It doesn't return discrete_coords. Which is a bit limiting.

E.g. if I want to quantize my cooardinates and additionally get unique map (to known which points were retained) it's not possible.
I can either get unique discrete coordinates (when calling with return_index=False) or unique_map (when calling with return_index=True).
But knowing unique_map only, I cannot easily compute discrete_coordinates. unique_map tells me which coords are retained (that's my understanding), but I need to apply unique_map index to my real coordinates and quantize them again, to get discrete coords. This is not optimal.

I think it'll be better, if the return_index flag returns unique_map and unique discrete coords. Discrete coords are computed within sparse_quantize, so they can be easily returned.

[chrischoy]

This was due to backward compatibility.

I originally used the quantization function only for returning indices and I agree that it would be more useful to return coordinates again as well.

I'll add this feature to the v0.5 TODO.