Add NIST based temperature calculation

[caternuson]

For #12. Adds a fancier temperature calculation which the older library had:
https://github.com/adafruit/Adafruit_Python_MAX31855/blob/master/Adafruit_MAX31855/MAX31855.py#L99

$ python3
Python 3.6.8 (default, Aug 20 2019, 17:12:48) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import board, digitalio, adafruit_max31855
>>> cs = digitalio.DigitalInOut(board.C0)
>>> spi = board.SPI()
>>> max = adafruit_max31855.MAX31855(spi, cs)
>>> max.temperature
25.5
>>> max.temperature_NIST
25.46810379959506
>>>

[ladyada]

by chance did you try running on circuitpy? no reason it shouldnt run but sometimes circuitpython is more constrained on something

[caternuson]

Didn't, but did just now:

Adafruit CircuitPython 4.1.0 on 2019-08-02; Adafruit ItsyBitsy M4 Express with samd51g19
>>> import board, digitalio, adafruit_max31855
>>> cs = digitalio.DigitalInOut(board.D7)
>>> spi = board.SPI()
>>> max = adafruit_max31855.MAX31855(spi, cs)
>>> max.temperature
25.5
>>> max.temperature_NIST
25.4659
>>>

and holding between fingers to generate something more than ambient:

>>> max.temperature, max.temperature_NIST
(30.25, 30.3106)
>>> max.temperature, max.temperature_NIST
(33.25, 33.3566)
>>> max.temperature, max.temperature_NIST
(34.0, 34.1181)
>>>

[ladyada]

good enough for government work!