Fix 3D transform (merges #75)

Take PR #75 and merge its changes to current master. Also fix some Python 3
related issues. Remove Transform3d from the ``dtcwt.opencl`` module for the
time being until it is fixed but keep the code in ``dtcwt.opencl.transform3d``
for ease of future merging.

TODO: automate generation of ``qbgn.npz`` otherwise next time ``gen_verif.m``
is run the tests will all fail.

Closes #75. Merges #75.