New issue 822: volume rendering classes are not imported in yt/__init__.py https://bitbucket.org/yt_analysis/yt/issue/822/volume-rendering-classes-are-...

Nathan Goldbaum:

Currently only `off_axis_projection` is imported. We should probably import `ColorTransferFunction`, `TransferFunctionHelper`, and any other names that people used to commonly import from `yt.mods`.