Kirsty Feijen

Rebased this as we have adapted the docs PR #5193

Do we want to expose the rest of the metadata classes/function in the API?

I checked through the code and this should be closed by #4191 Therefore, I am closing this now.

Any update here @adonath? Or should I try to implement this?

@registerrier and @adonath what is left to do with the clean up? We have this marked for 1.2

`select_nested_models` is now exposed (#4944 and #4961)

This should be covered now by https://github.com/gammapy/gammapy/pull/4810

@registerrier, I think this can be closed now? If not, I will add it to the new documentation issue I am creating!