Fixed PEP8 E301.
Fixed PEP8 E303.
tests: Mark failing tests as expected failures.

We'll fix them when we fix them.

Fixes #430.
world: Raise a different exception for impossible chunk lookups.

Some tests fail, but it's hard to see which ones. Y'know why? Because some of
the tests are putting random crap into the reactor and causing other tests to
fail later! Whoo.

Refs #423.
utilities/coords: Isolate iterneighbors(), and test.

Refs #423.
plugins/physics: Use iterneighbor().

Refs #423.
utilities/coords: Isolate itercube() and test.

Refs #423.
plugins/physics: Refactor to use itercube().

Refs #423.
plugins/automatons: Fix OOB coordinate access in Grass.

Fixes one misbehaving test.

Refs #423.
plugins/physics: Fix OOB access in Fluid.

Pervasive little bug.

Refs #423.
plugins/physics: Fix another OOB access in Fluid.

Fixes another test, and with this, all of the OOB accesses that we test for
are now fixed.

Fixes #423.
motd: Patches welcome!
chunk: Introduce a constant for chunk height.
Use CHUNK_HEIGHT in place of 256.
And update locations using 128 as well.
utilities/coords: Add a couple more iterators.

xz-coords are presented as a constant because they weigh just over 1KiB on
CPython and are computationally cheaper than an invocation of product() on
both CPython and PyPy.

CPython before: 34.6us
CPython after: 15.2us
CPython speedup: 2.28x
PyPy before: 146us
PyPy after: 1.29us
PyPy speedup: 113.18x
And use new coord helpers.

Tests lost about 4 seconds of runtime on PyPy. Encouraging!
plugins/window_hooks: Remove old print statements.
terrain/trees: Some PEP8.
terrain/trees: Share species numbers and use names for them.

Cleaner, faster, etc.