Force batch bins to fall on integer second boundaries

This PR changes behavior in utils.split_segments_by_livetime() so that generated folds always fall on integer second boundaries. The motivation behind this is that files written to disk have the T050017 file convention which don't have any finer resolution than 1 second, so that on the bin boundaries, timeseries may have a fraction of a second scattered across multiple files and may get overwritten as jobs from different bins complete. This may cause some missing data right at the boundaries which this addresses.