Virtual meta-snapshot file for distributed snapshots

@jch suggested creating a meta-file with links to the individual hdf5 files when writing a distributed snapshot. This could make the entire reading procedure more transparent for users as they would not have to worry about whether the snapshot is in 1 or N files.

A solution around virtual datasets could be interesting: https://support.hdfgroup.org/HDF5/docNewFeatures/NewFeaturesVirtualDatasetDocs.html