/** If the class does not exist, try to include it 
	 * If it could not be included, trigger a warning.
	 * @param string $className name of the class
	 * @param $dir application folder path
	 * For each class name inclusion is first tried from $thisfunction useClass(->getDir(), 
	 * 	    If the class is not found, a class from the classes root folder is tried.
	 *      However, this behavior may be overridden on Site::getTryUseClassTryParams
	 * @return boolean wheather a class file was included. Also true if the class already existed.
	 * @throw PntValidationException if no class included
	 */
	function useClass($className, $dir) {
		if (!$this->tryUseClass($className, $dir)) {
			$params = $this->getTryUseClassTryParams($className, $dir);
			throw new PntValidationException(
				"$this - useClass: class not found: "
					.$params[0][1]. $params[0][0].", "
					. $params[1][1]. $params[1][0]);
			return false;
		}
		return true;
	}