add arduino API

[hathach]

similar to adafruit/Adafruit_LSM6DS#21, this PRs add compatible API for arduino library

  int readMagneticField(float &x, float &y, float &z);
  float magneticFieldSampleRate(void);
  int magneticFieldAvailable(void);

PS: written fashion that requires adafruit/Adafruit_BusIO#46, therefore this is submitted as draft for now. Until the busio libraries release is indexed.