Add threshold functionality

[caternuson]

Adds ability to set low/high light thresholds and configure interrupt functionality.

Adafruit CircuitPython 3.1.2 on 2019-01-07; Adafruit ItsyBitsy M4 Express with samd51g19
>>> import board, busio
>>> from digitalio import DigitalInOut, Direction, Pull
>>> import adafruit_tsl2561
>>> i2c = busio.I2C(board.SCL, board.SDA)
>>> tsl = adafruit_tsl2561.TSL2561(i2c)
>>> pin = DigitalInOut(board.D10)
>>> pin.direction = Direction.INPUT
>>> pin.pull = Pull.UP
>>> tsl.luminosity
(74, 12)
>>> tsl.threshold_low = 20
>>> tsl.threshold_high = 1000
>>> tsl.cycles = 2
>>> tsl.interrupt_mode = 1
>>> pin.value
True
>>> tsl.luminosity
(1796, 212)
>>> pin.value
False
>>> tsl.clear_interrupt()
>>> pin.value
True
>>> tsl.luminosity
(27, 7)
>>> tsl.luminosity
(2, 1)
>>> pin.value
False
>>> 

[carterw]

Would please try your code with a lower threshold of zero and tell us what behavior you see? I was finding that an interrupt was generated when the lumens dropped under 5 even when the threshold was between zero and 4.

[caternuson]

Seems OK. When the interrupt is triggered, pin.value becomes False:

>>> tsl.interrupt_mode = 1
>>> tsl.threshold_high = 5000
>>> tsl.threshold_low = 0
>>> tsl.cycles = 1
>>> tsl.clear_interrupt()
>>> tsl.luminosity, pin.value
((65, 13), True)
>>> tsl.luminosity, pin.value
((4, 1), True)
>>> tsl.luminosity, pin.value
((7, 3), True)
>>> tsl.luminosity, pin.value
((0, 0), False)
>>> 

[tannewt]

Looks great! Thanks @caternuson