Tutorial (renaming - relabeling section): add a warning for set_axes()

[alixdamman]

Add a warning saying that set_axes() (set_axis_labels() in the future) does NOT check if new label make sense.

For example, if we have an Array 'pop' with an Axis gender=man,woman, the code:

pop = pop.set_axes('gender', 'women,men')

will not fail but values for men and women will be reversed.