Add column names and explanations to run_simulation() Roxygen2 documentation

[tbreweric]

I think it would be helpful to have each column of the model outputs explained in the documentation for the run_simulations() function so that there is something to refer to in the case of any confusion. I am not aware of any roxygen2 function for this purpose, but there is something analogous in malariasimulation we can borrow from.

[athowes]

Agree.