Fix error in infection flux of sir_typed.json

[djinnome]

$inf = \beta\frac{[S]\cdot[I]}{[S] + [I] + [R]}$

So,

"expression": "S*I*beta/(S+I+R)",

instead of

"expression": "S*I*beta",

[jpfairbanks]

The SIR example doesn't divide by N.

Model-Representations/petrinet/examples/sir.json

Line 88 in c0b064e

"expression": "S*I*beta",

There are no units in this model. So I don't this this is an error. I think this model is just written in terms of concentrations rather than populations.