Choose lead item for a set of samples.

    Picks tumors for tumor/normal pairs and first sample for batch groups.

def _pick_lead_item(items):
    """Choose lead item for a set of samples.

    Picks tumors for tumor/normal pairs and first sample for batch groups.
    """
    paired = vcfutils.get_paired(items)
    if paired:
        return paired.tumor_data
    else:
        return list(items)[0]