Add support for HTU21D/SHT21 temperature and humidity sensors

[pawel-sw]

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/en/*.

HTU21D is cheap digital temperature and humidity sensor.

[pjsg]

You don;t need the ICACHE_FLASH_ATTR as all nodemcu functions default to flash unless otherwise specified.

[pawel-sw]

thank you, I've removed the ICACHE_FLASH_ATTR

[devsaurus]

Please add I2C id as parameter to htu21.read() and check against platform_i2c_exists(). This seems redundant on ESP8266 platform, but helps future compatibility with ESP32.

[HHHartmann]

Hmm, this PR unfortunately is so old that it does not contain several changes we did. Pity that it wasn't picked up earlier.
Given that we now encourage pure Lua modules over C modules unless there is a reason for C (e.g. timing) this PR should be rewritten in Lua.
Same probably applies to #2261.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.