fixed reading archived data

Reading databases, which were rolluped, is broken. When ceres reader read data with retentin 5min, and then goes to slice with retention 60sec, it still try to use retention 5min. The "biggest_timeStep" is an attempt to solve this problem, first of all we have to get biggest timeStep for all slices, that has data for requests period. Second big issue is recalculating datapoint to the right (biggest) timeStep.