make sure hdf5 files always created with pytables
make sure hdf5 files always created with pytables

Otherwise, in python 3 we get this error:

https://github.com/pandas-dev/pandas/issues/9219

(and maybe things were silently weird in python 2 anyway).

Would probably be better to avoid using h5py at all, but the pytables
docs on the right way to do this are more work to understand than I feel
like right now.

dougalsutherland/pummeler