MMDA needs a pretty major refactor that will break some of its usage. They are:

1. A way of managing namespaces of different fields to allow for overloading (bib.title vs doc.title)
2. A way of .annotate() on a span_group rather than at a Document-level, for example, adding titles to bib entries
3. Making explicit annotation of BoxGroup from SpanGroup and defining explicit conversions from one to another