python3 update

[currymj]

Python 2 is going to be completely EOL starting January 1, 2020. Fortunately this repository is quite clean and has few dependencies, so automatically running 2to3 on this code resulted in code where all tests run and pass under Python 3.

Let me know if there's any other tests I should try to avoid subtle bugs, and whether or not a pull request for the Python 3 update would be worth submitting.

[currymj]

One minor change is that the main script needs to be run as:

python -m kidney_solver.kidney_solver 3 3 picef < example_data/MD-00001-00000100.input

instead of calling it directly, due to changes in the Python 3 import system.