Followed @astrofrog's suggestion and made these methods take the spectral_cube argument
Replacing pf with ds to reflect yt 3.0
Some unit tests for yt
Adding assert_array_equal for integer ndarrays
Most of these tests pass now, but there's a nasty bug that I need to track down.
These tests won't pass completely until yt 3.0.1 is installed in Anaconda.