Modify plot.get_ugf

This merge request closes #103 (closed).

-Now the function returns 4 values. The frequency, phase and gain at |G|=1, AND the index of the original (input) array closest to the ugf. -I stopped rounding the output, which I don't know why I was doing int he first place. -I modified the plot.critique function, to adjust for the changes above, as well as in order to better format the UGF information on the graphs.

Unfortunately, if anyone is already using get_ugf in their scripts, they will need to modify it, since there are now 4 values returned instead of 2. Maybe there is a way to preserve backwards compatibility, but I am not sure.