Helper function to display n/d and handle division by 0

Property definitions

counter :: counter $ Sys :: div
# Helper function to display `n/d` and handle division by 0
fun div(n: Int, d: Int): String
do
	if d == 0 then return "na"
	return ((100*n/d).to_f/100.0).to_precision(2)
end
lib/counter/counter.nit:390,1--395,3