Set infTime for 2-group/mode SIR stochastic models in closed pops

[smjenness]

Currently, this is set as a uniform over the rec.rate in the first group only, but should be group specific for differential recovery rates.

infTime[idsInf] <- ssample(1:(-round(1/all$param$rec.rate) + 2),
                                 length(idsInf),
                                 replace = TRUE)