Read frequency of calls from truth VCF.

    Currently handles DREAM data, needs generalization for other datasets.

def _read_truth_freqs(in_file):
    """Read frequency of calls from truth VCF.

    Currently handles DREAM data, needs generalization for other datasets.
    """
    out = {}
    with VariantFile(in_file) as bcf_in:
        for rec in bcf_in:
            freq = float(rec.info.get("VAF", 1.0))
            out[_get_key(rec)] = freq
    return out