Fix to flaw in event sequence for time-based simulations and minor improvement in STS digitizer.

The combination of the possibility to generate constant time intervals in time-based digitization (see !1204 (merged)) and the mixing of inputs can result in two MC events (from different inputs) being assigned the exact same event time (same in the sense of double precision). In such a case, one of the events would not be processed, since the event list to process is managed by a std::map with event time as key.

When sampling the event times with the random generator, this can practically never happen.

To cure the problem, the map was replaced with a multimap.

In the STS digitizer, setting the time resolution to a negative value will prevent calls to gRandom for sampling the digi time, which is a helpful debug feature.