Allow calibration of HuaFan devices (and other hardware)

[PeteBa]

The calibration limits hardcoded into xsns_hlw8012.ino are too high for calibrating the HuaFan devices. The HuaFan's shunt resistor is double the Sonoff's and there are only 4 voltage divider resistors rather than 5. For example, to calibrate the Huafan required the following values: HLWPCAL: 4,750, HLWUCAL: 1,525, HLWICAL: 1,680 (the first and last are below the hardcoded limits).

This PR proposes an alternative calibration approach allowing the user to provide known power, voltage and current values with the software auto-calibrating to these values. This would also allow the HuaFan to be calibrated as well as devices with other hardware. The PR adds the following three commands:

• HLWPSET x: where x is the expected power in watts i.e. 60 for a 60 watt bulb
• HLWUSET x: where x is the expected voltage in volts i.e. 240 for a 240 volt supply
• HLWISET x: where x is the expected current in milliamps i.e. 250 for 0.250mA

The use cases for calibration are either:

• a user connects up a known appliance, i.e. 60W bulb, and assumes 240V and 0.25A for voltage and current and enters these into the console
• a user connects a load and uses a multimeter to determine all three values

This approach seems more consistent with the guidelines provided in the wiki pages for preparation and simplifies the calibration process.

[pascalsaul]

The calibration is working quite nice with the SET command 👍 I'm only struggling with the voltage. Can it be outputted with one or two decimals? Because 1V is quite a big calibration gap. Now I need to wait when the voltage is rounded to calibrate.

Can you make an extra option to automatically calculate the PCAL when I have calibrated the U and I? Something like HLWPSETAUTO? I can measure the voltage (in parallel) and the current (in serie) with the multimeter. Calculating the P is quit hard with changing values :)

I don't believe that all bulbs have the exact same wattage ;) Thereby who got the old bulbs yet still?

When I measure exactly 231.0 V I send it to the device:
mosquitto_pub -t cmnd/pow_odu/HLWUset -m 231
18:24:43 MQTT: stat/pow_odu/RESULT = {"HlwUcal":"1944"}

When I look into the webinterface it keeps saying 230 V when my multimeter is saying 231.0

[PeteBa]

Thanks for adding this. Closing the pull request.