fix: axis metadata was broken when rebinning

[henryiii]

The metadata was getting an extra {"metadata": metadata} wrapped around it (and None was turning into {}). Seen in #977 (comment).

[henryiii]

As a followup, I think I'd like to rename .metadata on the C++ object, since it is not the same as .metadata on the Python one, and there's a high chance of confusion. It's actually the same as __dict__, which is likely a bad name, but maybe .raw_metadata or something like that would be less likely to be confused. Really anything that's not identical to .metadata. ;)

[henryiii]

Though it's called .metadata in C++. I wonder if we could just deprecate .metadata and metadata= (on the Python object) in favor of the newer .__dict__/__dict__=?