public static function accordions($atts, $content = null)
 {
     self::$accordionsCounters = self::$accordionsCounters > 0 ? (int) self::$accordionsCounters : 0;
     self::$accordionsCounters++;
     $content = do_shortcode($content);
     return '<div class="panel-group" id="accordion' . self::$accordionsCounters . '">' . $content . '</div>';
 }