volas-time: OHLCV time-frame cumulation (resampling) over the volas data
model. Groups fine bars on a DatetimeIndex into coarser periods and
aggregates each (open=first, high=max, low=min, close=last, volume=sum).
Depends on volas-core only — a sibling of volas-io / volas-directive
with no cross-sibling coupling.