def aggregation
      agg = DPLA::MAP::Aggregation.new(id)
      return nil unless agg.exists?
      agg.get
      agg
    end
def aggregation
      agg = DPLA::MAP::Aggregation.new(id)
      return nil unless agg.exists?
      agg.get
      agg
    end
Get the aggregation, populated with data from Marmotta, which corresponds
 to this SearchIndexDocument
 @return [DPLA::MAP::Aggregation, nil]