get_metadata() doesn't return a dict

[rgaudin]

get_metadata()'s docstring announces a returned dict with the file's metadata.

Instead, it expects a parameter, the metadata Key and returns a memory-view of the value for this key.

• we probably want str values (about all use cases) here as the raw content would still be available at r.get_article("M/Tags").content
• shall we have that dict of all defined meta somewhere? It would definitely be useful.
• docstring should be adjusted to the chosen behavior