How to use the pykern.pkio.save_chdir function in pykern

def _run_dose_calculation(data, cfg_dir):
    if not feature_config.cfg().rs4pi_dose_calc:
        dicom_dose = _run_dose_calculation_fake(data, cfg_dir)
    else:
        with pkio.save_chdir(cfg_dir):
            pksubprocess.check_call_with_signals(['bash', str(cfg_dir.join(template.DOSE_CALC_SH))])
            dicom_dose = template.generate_rtdose_file(data, cfg_dir)
    data['models']['dicomDose'] = dicom_dose
    # save results into simulation input data file, this is needed for further calls to get_simulation_frame()
    simulation_db.write_json(template_common.INPUT_BASE_NAME, data)
    simulation_db.write_result({
        'dicomDose': dicom_dose,
    })

def _run_hellweg(cfg_dir):
    with pkio.save_chdir(cfg_dir):
        exec(pkio.read_text(template_common.PARAMETERS_PYTHON_FILE), locals(), locals())
        pkio.write_text(template.HELLWEG_INPUT_FILE, input_file)
        pkio.write_text(template.HELLWEG_INI_FILE, ini_file)
        s = solver.BeamSolver(template.HELLWEG_INI_FILE, template.HELLWEG_INPUT_FILE)
        s.solve()
        s.save_output(template.HELLWEG_SUMMARY_FILE)
        s.dump_bin(template.HELLWEG_DUMP_FILE)

def run_background(cfg_dir):
    with pkio.save_chdir(cfg_dir):
        data = simulation_db.read_json(template_common.INPUT_BASE_NAME)
        res = {}
        if data.report == 'epicsServerAnimation':
            epics_settings = data.models.epicsServerAnimation
            if epics_settings.serverType == 'local':
                server_address = _run_epics(data)
                epics_settings.serverAddress = server_address
            else:
                assert epics_settings.serverAddress, 'missing remote server address'
                server_address = epics_settings.serverAddress
            _run_epics_monitor(server_address)
            if epics_settings.serverType == 'local':
                res['error'] = _run_simulation_loop(server_address)
            else:
                res['error'] = _run_forever(server_address)
        else:

def run_background(cfg_dir, sbatch=False):
    """Run code in ``cfg_dir`` with mpi

    Args:
        cfg_dir (str): directory to run code in
    """
    with pkio.save_chdir(cfg_dir):
        mpi.run_script(_script())
        simulation_db.write_result({})

def nginx_proxy():
    """Starts nginx in container.

    Used for development only.
    """
    assert pkconfig.channel_in('dev')
    run_dir = _run_dir().join('nginx_proxy').ensure(dir=True)
    with pkio.save_chdir(run_dir):
        f = run_dir.join('default.conf')
        values = dict(pkcollections.map_items(cfg))
        pkjinja.render_resource('nginx_proxy.conf', values, output=f)
        cmd = [
            'docker',
            'run',
            '--net=host',
            '--rm',
            '--volume={}:/etc/nginx/conf.d/default.conf'.format(f),
            'nginx',
        ]
        pksubprocess.check_call_with_signals(cmd)

def run(cfg_dir):
    """Run srw in ``cfg_dir``

    Args:
        cfg_dir (str): directory to run srw in
    """
    with pkio.save_chdir(cfg_dir):
        _run_srw()

def flower():
    """Start flower"""
    assert pkconfig.channel_in('dev')
    run_dir = _run_dir().join('flower').ensure(dir=True)
    with pkio.save_chdir(run_dir):
        command.FlowerCommand().execute_from_commandline([
            'flower',
            '--address=' + cfg.ip,
            '--app=sirepo.celery_tasks',
            '--no-color',
            '--persistent',
        ])