Feature/python3 -- resurrecting @wackywendell's python 3 support branch.

[joe-jordan]

A little while ago @wackywendell kindly made a couple of small changes for python three support and added some unit tests.

Here, I am finally adding these to the main repo. Will keep this as a PR while I check over the unit tests and code coverage.

[joe-jordan]

coverage gaps:

• numpy array inputs
• periodic boundary conditions
• with/without radii
• trigger voro++ error "number of cells not equal to the number of input particles" (I think this is triggered when a particle is on one of the walls, or outside the bounds?)

[joe-jordan]

Also, noticed that the Cython function has mutable arg defaults. It doesn't mutate them, but this should be fixed anyway.

[wackywendell]

Hi! Glad you were interested in doing this. Just thought I'd give you a heads up: I also wrote tess (docs here), which is another interface for voro++ with a more object-oriented interface. But its nice to also see pyvoro getting python 3 support!