USA and many other GAZ terms are classes & instances simultaneously

[yongqunh]

It appears that many GAZ terms like "USA" (http://purl.obolibrary.org/obo/GAZ_00002459) and "geographic location" (http://purl.obolibrary.org/obo/GAZ_00000448) are defined as both classes and instances. This may not be correct.

[beckyjackson]

There is a current discussion in #20 on this issue. I have proposed a solution but have not had any time to work on the implementation.