Tests from repository root.

[Bradley Dice]

We should be able to run tests from the repository root directory. Currently it tries to import the freud module directly, which won't work since it hasn't been compiled.

[Bradley Dice]

@vramasub I experimented with this for a while, trying to force freud to load the installed module if it detects the local directory. I was only moderately successful, and it was such a hack that I wouldn't feel comfortable changing the warning behavior we have currently. However, I would suggest we try running the tests directly from the tests/ directory instead of the home directory in CI. If we just cd tests/ and pass . as the path to nose, it should work fine and would be cleaner than our current setup.

[Vyas Ramasubramani]

Sure we can change that. I was looking a little into using setuptools to built cython with extension modules (that's what I've done with a package I wrote for a class), and I wonder if that might not be cleaner than our current build. The main difficulty with this would be incorporating libraries like TBB I would imagine. But it might be something we can look into as a general cleanup task. As a bonus, I think that would resolve this issue because that's the more standard workflow for cython repos.