Provide a useful file basename for outputs, referencing batch/sample and caller.

def _useful_basename(data):
    """Provide a useful file basename for outputs, referencing batch/sample and caller.
    """
    names = dd.get_batches(data)
    if not names:
        names = [dd.get_sample_name(data)]
    batch_name = names[0]
    return "%s-%s" % (batch_name, data["sv"]["variantcaller"])