fnv_hash_vec function used to downsample poiuntcloud has collisions on mirrored points

[Liquidmasl]

This might not be a problem if all points are always moved into the positive domain. but if not points that are mirrored sometimes collide.

1. 
Original: ([0.89963308, 0.30712294, 0.81770174])
Mirrored: ([-0.89963308, 0.30712294, 0.81770174])

2. 
Original: ([0.76068334, 0.49337516, 0.68670659])
Mirrored: ([-0.76068334, 0.49337516, 0.68670659])

3. 
Original: ([0.38544234, 0.94407035, 0.39409715])
Mirrored: ([-0.38544234, 0.94407035, 0.39409715])

... and so on.

anyway, this might lead to some issued if the points are not all put in the positive domain.

[guochengqian]

thanks for letting me know. I will look into this in next week.

[Liquidmasl]

we just added that to the top of the hash function, should be relatively fast and solves the issue as far as i tested it

# move all vertices to positive space
arr -= np.min(arr, axis=0)