In class Figure, graph -> pw

(For plot widget)
Classe Parameters cosmetics changes

Dics figures and curves are declared at the beginning of __init__ method of class Parameters
Class Window : Creation of method add_point

This method is useful to add a point to a curve.

It takes 3 parameters :
- The name of the curve
- The x of the point
- The y of the point
The main example plot 3 sines