Unzip .gz file. Return filename of unzipped file.

def gunzip(gzip_file, file_gunzip=None):
    """Unzip .gz file. Return filename of unzipped file."""
    if file_gunzip is None:
        file_gunzip = os.path.splitext(gzip_file)[0]
        gzip_open_to(gzip_file, file_gunzip)
        return file_gunzip