Feedback on Simplest_MD.ipynb

Hi @mrshirts ! I reviewed the first draft of Simplest_MD.ipynb via the [reviewNB ](https://www.reviewnb.com/)plugin. My comments are [here](https://app.reviewnb.com/shirtsgroup/simulation_essentials/blob/main/toy_problems%2FSimplest_MD.ipynb%2F/discussion). Let me know if you have any difficulty seeing them.