code: @Override
      public Set<URI> listServicesWithOutputType(URI modelReference) {
          return this.listEntitiesByDataModel(MSM.Service, MSM.hasOutput, modelReference);
      }
docs: Given the URI of a type (i.e., a modelReference), this method figures out all the services
  that have this as part of their inputs.
  
  @param modelReference the type of output sought for
  @return a Set of URIs of services that generate this output type.