Binning precision bug

[kevinTWD]

I think the offset in sort method should be added inside int(...) to correctly round ('floor', in fact) to the bin index. The current calculation is mathematically less precise, although it could be correct in many cases.

So the correction would be replacing

x = int(_[-3]/bin_interval) + offset
y = int(_[-2]/bin_interval) + offset
z = int(_[-1]/bin_interval) + offset

by

x = int(_[-3]/bin_interval + offset)
y = int(_[-2]/bin_interval + offset)
z = int(_[-1]/bin_interval + offset)