Fixed point iteration-non linear

It is a numerical method in solving two nonlinear equations of x and y. It is also called Method of Successive Substitution (MOSS) or simply Successive substitution. It plots the two functions to help the user decide which initial guesses of x and y to use. It asks the user the initial guesses for x and y and also lets the user choose what termination criterion to use, whether a specified % relative error or a number of iterations. It also checks if the system will totally converge or not and will alert the user if it would not totally converge.