Distributed HDF5 access

[cbyrohl]

We need to rework distributed hdf5 access once more, which appears to have stopped working.

TypeError: h5py objects cannot be pickle

There exists h5pickle, but this does not currently work as drop-in replacement. The issue appears to be related to DaanVanVugt/h5pickle#14.

A naive strategy would to provide only the filepath and hdf5 path to the dataset, opening/reading/closing the file for every dask chunk. The repeated opening and closing introduces a performance penalty. See discussion here