readme

<<Transnmission>>: The transmision event is combined for both strains since a person can be only infectious for one strain at a time

<<Infectiousness>>: updated at the time of contact (Population::modelContacts)

<<Immunity>>: we assume that both disease imly immunity toward reinfection with the same or the other disease. 0 by default, XX at the latency, 1 at the active disease and treament states.