Change axis start in single diode IV curve example

[RDaxini]

I think the graph in this example of modeling IV curves using a single-diode circuit equivalent model would look better if the x-axis (voltage) and y-axis (current) started at zero.

This is just a minor stylistic preference; it is more to serve as an easy issue that is worth holding off fixing so that it remains available for a new contributor to fix during the upcoming pvlib user group meeting in Copenhagen.

[cwhanse]

Or, add axis lines at x=0 and y=0