add more connections to the Uberon common_anatomy subset

[balhoff]

The Uberon taxon-independent common_anatomy subset is replacing the obsolete CARO ontology. It would be helpful to GO's usage of Plant Ontology terms if there were more links from PO to those Uberon terms. Could you add the following relations to PO?

• plant anatomical entity SubClassOf anatomical entity
• plant anatomical space SubClassOf anatomical space
• plant structure SubClassOf anatomical structure
• portion of plant substance SubClassOf organism substance

There might be other valid relations from PO to Uberon that I've missed. Instead of SubClassOf you may want to instead use a pattern like <po_term> EquivalentTo <uberon_term> and in_taxon some Viridiplantae if you think that makes sense. Thank you!