Retrieve calls, organized by name, to use for heterogeneity analysis.

def _get_calls(data, cnv_only=False):
    """Retrieve calls, organized by name, to use for heterogeneity analysis.
    """
    cnvs_supported = set(["cnvkit", "battenberg"])
    out = {}
    for sv in data.get("sv", []):
        if not cnv_only or sv["variantcaller"] in cnvs_supported:
            out[sv["variantcaller"]] = sv
    return out