Broken on Nvidia

It looks like the 3D tree is broken on more recent versions of the Nvidia driver.

I've tried a variety of driver versions:

• 367.x (now installed on porter): works
• 375.x: dies with unexplained "error -9999" on context creation on all tests after the first few
• 381.xx: same
• 384.xx (now installed on quail): Fails a bunch of tests

This is perhaps the simplest test that fails:

PYOPENCL_TEST=nvi python test_tree.py 'test_unpruned_particle_tree(cl._csc, np.float32, 3)'
Traceback (most recent call last):
  File "test_tree.py", line 1103, in <module>
    exec(sys.argv[1])
  File "<string>", line 1, in <module>
  File "test_tree.py", line 257, in test_unpruned_particle_tree
    do_plot=do_plot, max_particles_in_box=30, skip_prune=True)
  File "test_tree.py", line 125, in run_build_test
    debug=True, **kwargs)
  File "/home/andreask_work/src/boxtree/boxtree/tree_build.py", line 827, in __call__
    final_level_restrict_iteration)
AssertionError

It's not clear whether this is our or their fault, but it's not completely unreasonable that it might be ours, in which case it may be worth investigating.

cc @mattwala