MicroPython driver class for HMC5883L magnetometer, using @rm-hull 's Python code and ported to use pyb.I2C

Usage example:

>>>import hmc5883l
>>>compass = hmc5883l.HMC5883L(port = 2, gauss = "4.7", declination = (-2, 5))
>>>compass.heading()
34.45

• Uses continuous measurement mode. Single measurement mode can be easily implemented, though.
• You might want to tinker with 'gauss' value in constructor if you see that the output values do not correspond to the way sensor is moved.
• Beware of magnets - this driver does not do calibration =(

• http://magnetic-declination.com/Great Britain (UK) (dead link?)