Non-HDF5 backends for MetadorContainer

[apirogov]

The MetadorContainer interface does not depend on HDF5-specifics and could be extended to support, e.g.

• zip files
• plain directories (then it will just create the correct metadata directories in the right places)

To provide those, one just needs to define h5py-like access APIs that are translated into correct actions.

Some thought is needed for packers, that can be dependant on HDF5 features, so this would need some extra "supported archive" info that packers provide.

Packers that only use plain embedded files and simply attach metadata could be compatible with any backend.

[apirogov]

Moved this up a bit, because this could make Metador more attractive for metadata even if people don't care about HDF5 or even dislike it

[apirogov]

Changed my mind. Should do that only after at least one person asks about whether it works without HDF5, and that they'd prefer that.