Time-based simulations: Fixed ideal mode of sliding window seed finder

Added the class CbmTimeSlice to access the start time info. The Ideal mode of the sliding window seed finder is designed to use MC info about the event time as a seed time for the event builder. However, it is taking this info directly from MCEventList, where it is stored as the absolute time. The latter algorithm uses relative time for the event building. Digi times are not absolute, but they are relative to the start of the corresponding timeslice. Therefore, all seeds are being rejected because there are no digis in the window around the seedtime. To fix this issue, I have shifted the MC event times by the corresponding timeslice start times before storing them in the seedtimes vector.