def add(dcm)
      raise ArgumentError, "Invalid argument 'dcm'. Expected DObject, got #{dcm.class}." unless dcm.is_a?(DICOM::DObject)
      @dcm = dcm
      load_structures
    end
def add(dcm)
      raise ArgumentError, "Invalid argument 'dcm'. Expected DObject, got #{dcm.class}." unless dcm.is_a?(DICOM::DObject)
      @dcm = dcm
      load_structures
    end
Registers a DICOM object to the StructureSet, and processes it to create
 (and reference) the ROIs contained in the object.

 @param [DICOM::DObject] dcm an RTSTRUCT DICOM object