Preferably, these rails 8 a and 8 b are such as to have, for example, a length L of about 2 mm to 13 mm, a thickness ta of about 0.5 mm to 3 mm, the distance d between the rail 8 a and the rail 8 b being about 2 mm to 5 mm, a width wa of the upper rail 8 a being about 1 mm to 6 mm and a width wb of the lower rail 8 b being about 1 mm to 6 mm.