fix test: yt_version needs to be defined even if yt can't be imported
to_yt returns a YtCube object
pep8 -> package naming convention
fix tests: methods of correct objects
one more excess spectral_factor removed
use zero-indexed coordinates in yt2world/world2yt
allow 0 or 1 indexing in yt2world and back.  Clean their docs a little
use numpy >= 1.8
>= not allowed!
Merge pull request #129 from keflavich/yt_round2

yt continued: API shift