Fix standard form relabelling map for one-square origami

[slel]

Using surface_dynamics we can define the one-square origami:

sage: from surface_dynamics import Origami
sage: o = Origami('(1)', '(1)')
sage: o
(1)
(1)

For this origami,

• some methods work, such as o.cylinder_decomposition(), o.genus(), o.is_primitive()
• some methods fail, such as o.veech_group(), o.plot()

A special treatment in o._set_standard_form(return_map=True) for the one-square origami should fix that.

Indeed, this seems wrong:

sage: rel = o._set_standard_form(return_map=True)
sage: rel
(531920848,)