spike: multiparent ontologies [requires design discussion if this will be a new type]

[joerivandervelde]

Allow multiple parents for an ontology, to prevent adding the same term multiple terms (under different names) when there are multiple parents in the ontology inheritance

todo:

• check how hard to implement in UI components
• check if performance hit
• check if we want to introduce this as a new type (because we don't want to break old ontologies)

[mswertz]

will make @dtroelofsprins happy?

[chinook25]

Might be interesting to look at the directory ontologies, specifically the DiseaseTypes, because that used to be an ontology but was changed to support multiparents.