DS18B20 - CRC check added to lua module, minor optimization; change of ow module example

[vsky279]

No issue connected with this PR.

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I have 7 DS18B20 devices connected to 1 bus and I start to receive some wrong readouts. This may be connected to an hw issue. The module does not check CRC for readouts so wrong temperatures are reported. CRC check was added.

Beside this some minor optimization of the module.

This PR also changes nasty example in ow module documentation. The example was using tmr.delay(1000000)...

[HHHartmann]

Thanks for this contribution.
The build currently fails our new luacheck with the following messages:

   Checking lua_modules/ds18b20/ds18b20.lua          2 warnings

    lua_modules/ds18b20/ds18b20.lua:88:76: line contains trailing whitespace
    lua_modules/ds18b20/ds18b20.lua:102:43: line contains trailing whitespace 

[vsky279]

It's great we have such check. Corrected.