Should it be possible to display the expectation value in the Single Photon mode?

[jbphet]

As of this writing, the design document doesn't show the "Expectation Value" checkbox in the "Single Photon" mode - it's only shown for the "Many Photons" mode. I asked in yesterday's design meeting if we might want to allow the user to see the expectation value in the single-photon mode, since it's kind of cool to see the graph converge to it as the number of samples increases. We discussed pros and cons of this, and ultimately decided to show it in both modes and run the question by our consulting teachers when we get a chance. I've made this change and will mark the related code with a TODO that references this issue.

[jbphet]

It's set up to show in both modes and the TODO is in the code. Assigning to @arouinfar for follow-up with our consulting instructors.