saveAs( filename, format ) {
    if (format === 'JRS') {
      this.imp.file = filename || this.imp.file;
      FS.writeFileSync( this.imp.file, this.stringify(), 'utf8' );
    } else {
      const newRep = CONVERTER.toFRESH(this);
      const stringRep = CONVERTER.toSTRING(newRep);
      FS.writeFileSync(filename, stringRep, 'utf8');
    }
    return this;
  }
Save the sheet to disk in a specific format, either FRESH or JRS.