Define proper way of passing Drug type info from Reactome in BioPAX

[dustine32]

Before we were relying on a chemical having an xref to "Guide to Pharmacology" to detect whether it's a drug. Some of the enities in Reactome are type Drug but do not have this xref. It would be great to somehow export this Reactome type info into the BioPAX so we can consistently match Reactome drug status for an entity.

Ex: carbovir monophosphate [cytosol] R-ALL-2161176

Originated from #264

[dustine32]

From Guanming at Reactome:

I think we may hack by adding a new bp:comment for drugs like the following (look at the first coment):

  <bp:SmallMolecule rdf:ID="SmallMolecule15">
    <bp:comment rdf:datatype="http://www.w3.org/2001/XMLSchema#string">Converted from Drug in Reactome</bp:comment>
    <bp:displayName rdf:datatype="http://www.w3.org/2001/XMLSchema#string">xenobiotic</bp:displayName>
    <bp:name rdf:datatype="http://www.w3.org/2001/XMLSchema#string">carbovir monophosphate</bp:name>
    <bp:cellularLocation rdf:resource="#CellularLocationVocabulary1" />
    <bp:entityReference rdf:resource="#SmallMoleculeReference15" />
    <bp:comment rdf:datatype="http://www.w3.org/2001/XMLSchema#string">Reactome DB_ID: 2161176</bp:comment>
    <bp:xref rdf:resource="#UnificationXref64" />
    <bp:xref rdf:resource="#UnificationXref65" />
    <bp:dataSource rdf:resource="#Provenance1" />
    <bp:xref rdf:resource="#RelationshipXref19" />
  </bp:SmallMolecule>