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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _prep_cnv_file(in_file, work_dir, somatic_info):
 """Prepare Battenberg CNV file for ingest by PhyloWGS. The PhyloWGS preparation script does not handle 'chr' prefixed chromosomes (hg19 style) correctly. This converts them over to GRCh37 (no 'chr') style to match preparation work in _prep_vrn_file. """

    out_file = os.path.join(work_dir, "%s-prep%s" % utils.splitext_plus(os.path.basename(in_file)))
    if not utils.file_uptodate(out_file, in_file):
        with file_transaction(somatic_info.tumor_data, out_file) as tx_out_file:
            with open(in_file) as in_handle:
                with open(tx_out_file, "w") as out_handle:
                    out_handle.write(in_handle.readline())  # header
                    for line in in_handle:
                        parts = line.split("\t")
                        parts[1] = _phylowgs_compatible_chroms(parts[1])
                        out_handle.write("\t".join(parts))
    return out_file