Add option to include average star/model in StarImages plots

[rmjarvis]

When thinking about useful plots for a DE School lesson I'm doing with @arunkannawadi, I realized it would be nice to show the average star and PSF image, rather than just a sampling of them as we currently do in the StarImages plot.

This PR adds an option to include this average image as the first plot in a StarImages plot. The default for this option is True, but you can turn it off with include_ave=False.

Here is an example of what the output looks like now with the average plot included: