Retrieve small variant (SNP, indel) VCFs compatible with CNVkit.

def _compatible_small_variants(data, items):
    """Retrieve small variant (SNP, indel) VCFs compatible with CNVkit.
    """
    from bcbio import heterogeneity
    VarFile = collections.namedtuple("VarFile", ["name", "sample", "normal"])
    out = []
    paired = vcfutils.get_paired(items)
    for v in heterogeneity.get_variants(data, include_germline=not paired):
        vrn_file = v["vrn_file"]
        base, ext = utils.splitext_plus(os.path.basename(vrn_file))
        if paired:
            out.append(VarFile(vrn_file, paired.tumor_name, paired.normal_name))
        else:
            out.append(VarFile(vrn_file, dd.get_sample_name(data), None))
    return out