def add(substance,feature,value)
      substance = substance.id if substance.is_a? Substance
      feature = feature.id if feature.is_a? Feature
      data_entries[substance.to_s] ||= {}
      data_entries[substance.to_s][feature.to_s] ||= []
      data_entries[substance.to_s][feature.to_s] << value
      #data_entries[substance.to_s][feature.to_s].uniq! if value.numeric? # assuming that identical values come from the same source
    end
def add(substance,feature,value)
      substance = substance.id if substance.is_a? Substance
      feature = feature.id if feature.is_a? Feature
      data_entries[substance.to_s] ||= {}
      data_entries[substance.to_s][feature.to_s] ||= []
      data_entries[substance.to_s][feature.to_s] << value
      #data_entries[substance.to_s][feature.to_s].uniq! if value.numeric? # assuming that identical values come from the same source
    end
Writers
 Add a value for a given substance and feature
 @param [OpenTox::Substance,BSON::ObjectId,String] substance or substance id
 @param [OpenTox::Feature,BSON::ObjectId,String] feature or feature id
 @param [TrueClass,FalseClass,Float]