public function entrar()
 {
     if (isset($_POST['submit'])) {
         $login = new LoginModel();
         $login->username = $_POST['username'];
         $login->contrasena = $_POST['contrasena'];
         if ($login->logar()) {
             $_SESSION['usuario'] = $login->logar();
         }
         header('Location:index.php');
     }
     include 'View/Site/Login.php';
 }