Make interval exchange transformations work with permutation with flips

[videlec]

The current interval exchange transformation objects (in surface_dynamics/interval_exchanges/iet.py) do not handle flips. It is not too complicated to modify the current class IntervalExchangeTransformation to handle them. Here is list of methods that should be modified

• translations
• erase_marked_points
• __mul__ (for composition)
• __call__ (for computing image of a point)
• rauzy_move, backward_rauzy_move, zorich_move
• plot_functions, plot_two_intervals, plot_towers

[magalijay]

I have written a new version of __mul__ and __call__ and a new method flipped_translations.

[videlec]

Super! Est-ce que tu sais comment faire une "pull request"? Sinon, on peut aussi discuter sur zulip.

[videlec]

To fit with sage-flatsurf terminology, maybe these should be called "half-translation". Even though I am not a big fan of this terminology.