Sort is missing as part of simplification

[BracketJohn]

This currently does not hold:

def test_simplify_order(are_asts_equal):
    """Test whether order is also irrelevant."""
    ast_one = Node(gpflow.kernels.Sum, full_name='Sum')
    Node(gpflow.kernels.RBF, parent=ast_one, full_name='rbf')
    Node(gpflow.kernels.Constant, parent=ast_one, full_name='constant')

    ast_two = Node(gpflow.kernels.Sum, full_name='Sum')
    Node(gpflow.kernels.Constant, parent=ast_two, full_name='constant')
    Node(gpflow.kernels.RBF, parent=ast_two, full_name='rbf')

    simpl_one = simplify(ast_one)
    simpl_two = simplify(ast_two)

    assert are_asts_equal(simpl_one, simpl_two)

To fix this:

• sort has to be implemented,
• conftest.py::are_asts_equal has to get the following addition:

       	if lvl_order_ast_one != lvl_order_ast_two:
   	        return False
  

  This could be part of another if block, maybe we don't need to check order every time.