Remove index shift by +-0.5 in voxel.py

[wkentaro]

Close #370

NOTE: This is not backward compatible.

[mikedh]

Nice! This was sort of haphazard before, it's great to make it consistent. A few quick thoughts:

• could we unit test?
• are there other places do we do the conversion points->indices? We should probably be consistent
• Maybe a quick blurb in the voxel.py top docstring which says something like "all voxels are referenced by the center of their cell box"

[wkentaro]

could we unit test?

I fixed the unit test just now.

are there other places do we do the conversion points->indices? We should probably be consistent

As far as I know, the conversion is only used in voxel.py file. So I think now is good.
The only concern is someone can assume Voxel.as_boxes returns boxes in the previous coordinate system. (origin is right-top of the first box)

Maybe a quick blurb in the voxel.py top docstring which says something like "all voxels are referenced by the center of their cell box"

I agree. I added a docstring for the Voxel class.

[coveralls]

Coverage increased (+0.02%) to 84.002% when pulling 2d5e969 on wkentaro:remove_index_shift_voxel into dd0225c on mikedh:master.

[mikedh]

Awesome, thanks!