Issue 12: remove equivalents as subclasses

[wdduncan]

Fixes #12
The nmco-classes.json file had equivalent classes as subclasses of each other (i.e., X is_a Y and Y is_a X).

This is confusing for consumers of the data. These subclass statements have been removed for equivalent classes. Equivalent class information is now in the equivalentNodesSets section of the file. E.g.:

"equivalentNodesSets" : [ {
      "nodeIds" : [ "http://purl.obolibrary.org/obo/ENVO_01001784", "http://purl.obolibrary.org/obo/ENVO_01001479" ]
    }, {
      "nodeIds" : [ "http://purl.obolibrary.org/obo/FOODON_00001100", "http://purl.obolibrary.org/obo/FOODON_03301103" ]
    }, {
      "nodeIds" : [ "http://purl.obolibrary.org/obo/FOODON_03460231", "http://purl.obolibrary.org/obo/FOODON_03460229" ]
    } ]