Providing for custom shunt resistors

[Bogdancev]

I had to de-solder 0.1 Ohm resistor and put 0.04 in. Now I need to change the code of the library to make it work. As alternative could you change assess modifier like so?:

protected:
    Adafruit_I2CDevice *i2c_dev = NULL;
    uint8_t ina219_i2caddr = -1;
    uint32_t ina219_calValue;
    // The following multipliers are used to convert raw current and power
    // values to mA and mW, taking into account the current config settings
    uint32_t ina219_currentDivider_mA;
    float ina219_powerMultiplier_mW;

private:
    void init();
    int16_t getBusVoltage_raw();
    int16_t getShuntVoltage_raw();
    int16_t getCurrent_raw();
    int16_t getPower_raw();

Protected fields are accessible in child class and thus I could use it like so:

class MyINA219 : public Adafruit_INA219 {
  public:
  void setCalibration_32_5A() {
    
    ...
    
    ina219_calValue = 5012;
    ina219_currentDivider_mA = 20; 
    ina219_powerMultiplier_mW = 4;
   
    ...
    
  }
};

Note, that this allows to add custom calibration methods to accommodate any resistor values.
This change will allow for more flexibility.
I can add Pull Request should you accept this change.

[xitonghan]

I=u/r
ShuntVoltage you can get R you know
so I is easy
and W=UI so ..