make kits21 a python package

[FabianIsensee]

make kits21 a python package

Description

restructuring of existing code to make KiTS a python package. This will allow participants to import the code more easily into their own projects (for example the metric computation and ranking code which we expect participants will be using to evaluate their methods during development).

Checklist

• [x] Merged latest master
• [x] Updated version number in README.md
• [ ] Ran annotation.import to completion -> cannot do this because I don't have the necessary data
• [x] Added changes to changelog.md

[neheller]

This makes sense. I'm going to merge it into a dev branch within my fork so that I can more easily make some small changes.