Consider switching to "src" package layout and using "tox" for test running

When hacking on Kwant it is recommended by the Kwant docs to use python setup.py build_ext --inplace && pytest for testing.

There are a couple of problems I have observed with this workflow:

1. the build_ext --inplace step is kind of non-standard, and if done incorrectly (e.g. by omitting the --inplace) then python -c 'import kwant' complains about not importing Kwant from the correct place (this has confused me more times than I care to mention)
2. placing the .c files next to the source an be problematic. Cython is not re-run if the associated .pyx file has not changed, but if we are using different versions of Cython, or targeting different CPython APIs (can happen when jumping around between stable/master) then the .c files are still not regenerated.
3. locally testing against different environments becomes a pain (manually deactivating / reactivating different Conda environments / whoops I just wasted 10 minutes because I was in the wrong environment)

I believe that points 1 and 3 can be addressed by using the src layout (i.e. have the Kwant source code under src/kwant rather than directly in kwant) and by using an environment-aware test-runner such as tox (which also has support for Conda environments).

I am not sure if this will address 2, but 2/3 ain't bad.