Determine if this call feeds downstream into joint calls.

def _joint_calling(items):
    """Determine if this call feeds downstream into joint calls.
    """
    jointcaller = tz.get_in(("config", "algorithm", "jointcaller"), items[0])
    if jointcaller:
        assert len(items) == 1, "Can only do joint calling preparation with GATK with single samples"
        assert tz.get_in(("metadata", "batch"), items[0]) is not None, \
            "Joint calling requires batched samples, %s has no metadata batch." % dd.get_sample_name(items[0])
    return jointcaller