Add Calibration/Offset about atc_mithermometer HOT 6 CLOSED

SHTV3/SHTC3 sensor is pretty accurate according to datasheet, so there should be no need for calibration I think. RH measurement is not so simple/straightforward (as temperature for example), so consider it as approximate value like too dry - good - to moist :)

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In my experience (and also from what I have read, the lywsd03mmc has a tendency to show humidity 2%-5% above the actual value (not sure if that's the sensor or something Xiaomi did with the board). Having a fixed offset would be nice to mitigate this (even if the error is not 100% linear, it should still be more accurate).

I would do it in my smart home implementation but I don't like the idea of having different value on the display and within my smart home.

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It coulde be added and then writen to the flash. Need to experiment with flash writing a bit as it needs thinking about layout and position. Then it could set and read via a characteristic.

If its usefull or not is not so important i think as long as it helps the user experience.
for me the humidity is something i would not trust on its very position dependend as a simplefinger near by is already changeing the values.

What i did see is that the xiaomi value is a bit different to the custom one ca bee seen in the video, the custom one against each other are very much the same values

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That might be a good observation. Maybe the Xiaomi firmware does some computing on the values, messing them up. I guess it makes sense to compare this a bit closer before implementing something.
I'll place two devices next to each other and compare values for a while.

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Ok so that's interesting. While the temperature stayed the same after flashing, the humidity dropped by 5% - So whatever the original firmware is doing, it doesn't seem does seem to alter the humidity quite a bit.

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Added an offset for Temp and Humidity,

Temp from -12,5 - +12,5 °C

Humi -50 - +50%

The offset will be parsed as int_8 so an 0x80 would be -12,5°C something.

https://github.com/atc1441/ATC_MiThermometer#temp-and-humi-offset

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