Plot background light curve in S3/S4 as another check for self-subtraction

[taylorbell57]

Not a recommendation for this PR, but we should really consider making this plot for spectroscopy data as well

Originally posted by @taylorbell57 in #437 (comment)

We should make a plot in S3/S4 which shows the temporal evolution of the background as a check that the background is not dominated by the source light. In Sebastian's PR example plot, it is clear that there is a transit in the background flux lightcurve which demonstrates that at least some self-subtraction is happening.