make (sum of two gaussians) plot with user bindings

[bertdv]

Please update this plot of two Gaussians #Code-Example:-Sum-of-Two-Gaussian-distributed-Variables

1. All julia code should be hidden. At this point we have not yet introduced Julia in the course.
2. It should just be the plot with sliders for the variables (mean and variance), so the user can play with the two Gaussians that are added and review the resulting sum distribution.