code: public String getSource() {
  		if (source != null) {
  			return source;
  		}
  		if (parentSequence != null) {
  			return parentSequence.getSource();
  		}
  		return null;
  	}
docs: Added support for the source of this sequence for GFF3 export
  If a sub sequence doesn't have  source then check for parent source
  @return the source