HDF5 input and output

[swenger]

It's not immediately clear how DataArray instances should be represented in HDF5:

http://fperez.github.io/datarray-doc/0.0.6/design_questions/issues.html#how-do-i-save-a-datarray-in-hdf5-using-h5py

However, there seems to be an official standard for storing axis dimensions and labels in HDF5 that apparently maps quite naturally to DataArray:

http://www.hdfgroup.org/HDF5/doc/HL/H5DS_Spec.pdf

Since the specification proposes to store the metadata in regular HDF5 datasets, this could probably implemented in h5py without resorting to the raw HDF5 API.

[rainwoodman]

Please please take the time to properly support vector columns. I believe recent versions of h5py can correctly write them.