Calculate the final size of an epidemic.
Usethe the following logic:

## Arguments

- r0
a double, the reproduction number (can be basic or Reff)

## Details

1 + 1/R0*W(-R0exp(-R0))

## Examples

```
if (FALSE) {
epi_finalsize(3.5)
plot(seq(1,3,.01), epi_finalsize(seq(1,3,.01)),xlab = "R",
ylab = "Percent Infected", type = "l")
}
```