Euler's method

It is the simple Euler's method, an iterative approach in finding the y value for a given x value starting from a 1st order ODE. It asks the user the ODE function and the initial values and increment value. It also lets the user choose what termination criterion to use, either a specified x value or a number of iterations. If the solution curve for the given ODE is readily available, the user can input it and have the true %relative error.