def create_epoch(f, name, start_time, stop_time):
    """ add an <epoch_X>.  f is the h5gate file object.
    name is name of epoch
    start_time and stop_time should both be float64
    Returns the group for the created epoch.
    """
    epoch = f.make_group("<epoch_X>", name)
    epoch.set_dataset("start_time", start_time)
    epoch.set_dataset("stop_time", stop_time)
    return epoch