Now saving numpy arrays completely overwrites the hdf5 file.
Now saving numpy arrays completely overwrites the hdf5 file.

Fixes #20. It would be nice to find a less destructive fix, however.

datreant/datreant.data,datreant/datreant.core,datreant/datreant,datreant/datreant,dotsdl/datreant,datreant/datreant.core