Add support for load cell / weight sensors

[ghost]

Currently, there is no support for Load cell or weight sensors in the Sensor type and standardized SI values. Is there a plan to add support or open to contributing to add support?

I am looking to add support for NAU7802 to wippersnapper

[brentru]

Open to contributions and pull requests, feel free to tag me!

[tyeth]

Leaving this open for revisiting as we add the HX711 in the near distant future.
Currently we are adding the NAU7802, but there is not yet the ability to set gain, or zero offset (TARE), or calibration offset, so those features would be required before some SI units could be utilised.
There was a discussion when adding the NAU7802 about having a raw force unit, adafruit/Adafruit_Wippersnapper_Arduino#577 (comment)
but not sure if that's better than just a unitless_float type that would encompass non SI things.