Moderate changes the decision tree related to the additive model & use of th Previous button & in messages

Continued improvement of the elimination of the question related to large fluctuation in abundance when overexploitation if there is no corresponding (additive) model available.
Improvement of plots titles.
Addition of a message about the use of the Previous button.
Addition of a message related to MSY & MSE graphs display.
Transformation from integer to boolean of the variables Flag_Modele_Lineaire_deja_recherche, Flag_Modele_Exponentiel_deja_recherche
Improvement of message contents related to the stop of the procedure according to R2 values.