def default_derived_characteristics
      chrs = specimen_characteristics || return
      pas = chrs.class.nondomain_attributes.reject { |pa| pa == :identifier }
      chrs.copy(pas)
    end
def default_derived_characteristics
      chrs = specimen_characteristics || return
      pas = chrs.class.nondomain_attributes.reject { |pa| pa == :identifier }
      chrs.copy(pas)
    end
Returns characteristics to use for a derived specimen. The new characteristics is copied from this
 parent specimen's characteristics, without the identifier.

 @return [CaTissue::SpecimenCharacteristics, nil] a copy of this Specimen's specimen_characteristics, or nil if none