def print
      @patients.each do |p|
        puts p.name
        p.studies.each do |st|
          puts "  #{st.uid}"
          st.series.each do |se|
            puts "    #{se.modality}"
            if se.respond_to?(:images) && se.images
              puts "      (#{se.images.length} images)"
            end
          end
        end
      end
    end
def print
      @patients.each do |p|
        puts p.name
        p.studies.each do |st|
          puts "  #{st.uid}"
          st.series.each do |se|
            puts "    #{se.modality}"
            if se.respond_to?(:images) && se.images
              puts "      (#{se.images.length} images)"
            end
          end
        end
      end
    end
Prints the nested structure of patient - study - series - images that
 have been loaded in the DataSet instance.