simplify UmbrellaClassifierData API

based on a discussion in !67 (merged), @patrick.godwin suggests

Since adding these, would you want to just make extend_child and strip_child the only ways to add/remove CDs into the umbrella and remove the rest/make them private? It would clean up the API and bit and have guaranteed behavior.

This would require modifying factories.QuiverFactory to avoid using append_child, but it seems worth it in the long run.