NF: Draft of export/import functionality for long-term dataset storage

This is not complete -- a proposal for discussion.

Idea: With our serialization magic, h5save() is not a good format approach for storing a dataset in the long-term.
This proposal provides two functions that can load/save datasets into NumPy's NPZ format (compressed/uncompressed). Naturally only samples, sa and fa collections are stored, hence no mappers or other attributes.

Documentation will be completed once discussed.