Adding 1st example contrib'd from https://developers.google.com/webmasters/business-location-pages/schema.org-examples
added
       <span>Subclass of: <a property="rdfs:subClassOf" href="http://schema.org/Enumeration">Enumeration</a></span>
... to DrugCostCategory.

See also
https://github.com/rvguha/schemaorg/pull/14
https://github.com/rvguha/schemaorg/pull/13
Merge branch 'master' of https://github.com/rvguha/schemaorg
Each MedicalEnumeration is now also explicitly asserted to be an Enumeration.
This is because our site code does not currently do type hiearachy reasoning when
deciding whether to list the instances of an enumeration.