Include a scenario graph in the report

This basic graph is generated from the model and is the same as the one in the expert app.

It doesn't have a fixed y scale (something that has been requested). This can be done subsequently (cc @jdevine to define in a separate issue).

Sample graph:

Currently worth noting @mrognlie and @nimounet is that the model was originally time-invariant and ran off a hours-from-start time definition. Since the introduction of time-based ventilation schemes we now encode the time (not hour from start) in the model, and we have to be very careful with the concentration calculations with that. I currently handle that in this MR by plotting the interval that the infected (should be exposed but doesn't yet exist) is in the room.