Compatibility fix for h5py 2.2

H5MD uses undocumented functions from h5py._hl. This fix makes it
work with h5py 2.2 in addition to whatever it worked with before.
In the long run, the generation of compact datasets should probably
use the documented low-level API instead.
Merge pull request #2 from khinsen/h5py_2.2

Compatibility fix for h5py 2.2