Customize child links

[Matthias Mohr]

If I add a child link to a catalog, there's no easy way to customize it (i.e. add an additional property). The created Link object is completely hidden from the users and getting it via a get_child_links or similar fundtions is cumbersome.

I could imagine two ways:

1. Add an extra_fields parameter to add_child
2. Return the created link object (instead of None) so that we can add more fields to the link

Thoughts? Happy to create a PR, but would like to get some thoughts from the maintainers first.

[Pete Gadomski]

Return the created link object (instead of None) so that we can add more fields to the link

This feels the most intuitive to me.

[Matthias Mohr]

Thanks, I'll create a PR that applies this behavior.