Check for multiple parents

[joncison]

Raise WARN but we need to define exceptions according to the patters in EDAM where we know this is OK, e.g.

• Formats link to both Format (by type of data)(http://edamontology.org/format_2350) (or kids) and one of XML, Textual format etc.
• Operations can link to a top-level Operation in addition to others
• etc. (we need to enumerate exact rules)

Maybe raise ERROR when something has more than two parents - this points to a conceptual mess that needs cleaning up.

I'll firm this up once the Editors Guide (http://edamontologydocs.readthedocs.io/en/latest/editors_guide.html) is done.