Apply calibration values from `fit_fb_calibration` result to `calibration`
    object.

def apply_calibration(df, calibration_df, calibration):
    '''
    Apply calibration values from `fit_fb_calibration` result to `calibration`
    object.
    '''
    from dmf_control_board_firmware import FeedbackResults

    for i, (fb_resistor, R_fb, C_fb) in calibration_df[['fb_resistor', 'R_fb', 'C_fb']].iterrows():
        calibration.R_fb[int(fb_resistor)] = R_fb
        calibration.C_fb[int(fb_resistor)] = C_fb

    cleaned_df = df.dropna()
    grouped = cleaned_df.groupby(['frequency', 'test_capacitor', 'repeat_index'])

    for (f, channel, repeat_index), group in grouped:
        r = FeedbackResults(group.V_actuation.iloc[0], f, 5.0,
                            group.V_hv.values, group.hv_resistor.values,
                            group.V_fb.values, group.fb_resistor.values,
                            calibration)
        # Update the measured capacitance values based on the updated
        # calibration model.
        df.loc[group.index, 'C'] = r.capacitance()