Update plots as simulations run

[connor-french]

Need to figure out how to update plots as the simulation outputs at different checkpoints. Can likely do this in a reactive context, but may need to rely on the future R package.

• figure out with base R
• Add interactivity -possibly rewrite in JavaScript (may open another issue for this when we get there)

[connor-french]

Turns out the callr R package with the r_bg() function seems to work well. It sends the expensive process to a background R session and does it in a way that the Shiny user can still interact with the app while the process is running. This is distinct from future and promises, which would require you to wait while the process runs. It also doesn't seem to rely on multi-threading, which could reduce the chance of bugs across operating systems.