Add FamilyTree diagram type

A new diagram type for creating actual family trees would be useful for geneological work. This could be mostly a subclass of TreeDiagrams. But it will need some added complexity to support pairs of parents as parent nodes.

Start by locating the existing TreeDiagrams implementation and related diagram types in the Python library. Study how parent nodes are represented and how diagrams are generated. Done means a FamilyTree diagram type exists, supports pairs of parents as parent nodes, and can create family-tree diagrams for genealogical work.

Written by the indexing model from the issue text.