modes: RES: ad hoc specification of threshold levels and splitting

In the modes commands for rare event simulation, make --levels accept a string of pairs "t1,s1 t2,s2 ... tn,sn" (in whatever syntax is convenient). All pairs should be numbers: "ti,si" means that the i-th threshold used for RES is set at importance value "ti," and that the splitting to perform there is "si."