add dark frame calibration and add calculateLux method as per Si114x AN523

[gtalusan]

... aka make the SI1145 more useful on the Arduino.

The SI1145 datasheet is quoted as saying:

Accurate lux measurements with IR correction algorithm

It's hard to determine exactly what the algorithm is until looking at their SI1145 AN523 application note. The lux calculation is given as such:

Lux level = [ [(ALS visible reading) - (ALS visible dark reading)] x (ALS visible coefficient) +
[ (ALSIR reading) - (ALS IR dark reading)] x (ALS IR coefficient) ] x gain correction

The dark readings are absolute minimums reported by the sensor in "absolute" darkness, and the coefficients are gleaned from the table in AN523.6. I have changed the Adafruit_SI1145 class to keep track of the absolute minimums of the VIS and IR channels and use them for dark frame subtraction. The initial dark values were determined by covering the sensor with a black cloth.

I have also exposed an API for adjusting the gain on the visible and IR channels.

I have a TSL2561, TSL2591 and SI1145 hooked up to my Arduino Uno's I2C bus and the EV calculations are in relative agreement.

TSL2561 IR: 88  Full: 228 Lux: 94.00  EV: 3.63
TSL2591 IR: 94  Full: 232 Lux: 63.517436  EV: 3.24
SI1145 IR: 14  Full: 12 Lux: 63.799999  EV: 3.24

TSL2561 IR: 86  Full: 225 Lux: 94.00  EV: 3.63
TSL2591 IR: 93  Full: 227 Lux: 60.775676  EV: 3.19
SI1145 IR: 14  Full: 13 Lux: 69.209999  EV: 3.32

TSL2561 IR: 83  Full: 219 Lux: 93.00  EV: 3.62
TSL2591 IR: 90  Full: 221 Lux: 59.894393  EV: 3.18
SI1145 IR: 13  Full: 13 Lux: 69.290000  EV: 3.32