Add SHT20 using si7021 driver

[tyeth]

Adds SHT20

[brentru]

@tyeth This should be fine. Have you tested it on hardware yet?

[tyeth]

Yes just this second seen another bug though, #410, it seems to not send a Device Init message when you add a Dev component, but it works with a reboot (the component is added to the list but the device gets no component Init message).
This can now be nicely tested with the Si7021 and the SHT20 (both use same driver but one is Dev state). I believe the HU21D is also driver compatible if you have one of those and no Si7021/SHT20.

An example of the console output for deleting the Si7021 and adding it as the SHT20 instead. It de-inits the si7021, does a scan when adding the SHT20 to verify address, then never does an Init. The same is not the case with non-dev components.

17:45:19.343 -> PING!
17:45:24.084 -> PING!
17:45:28.767 -> PING!
17:45:30.201 -> * NEW MESSAGE [Topic: Signal-I2C]: 
17:45:30.201 -> 4 bytes.
17:45:30.201 -> cbDecodeSignalRequestI2C
17:45:30.201 -> NEW COMMAND: I2C Device Deinit
17:45:30.201 -> Publishing Message: I2CResponse...Published!
17:45:33.492 -> PING!
17:45:38.205 -> PING!
17:45:42.908 -> PING!
17:45:45.788 -> * NEW MESSAGE [Topic: Signal-I2C]: 
17:45:45.788 -> 12 bytes.
17:45:45.788 -> cbDecodeSignalRequestI2C
17:45:45.788 -> I2C Scan Request
17:45:45.788 -> EXEC: I2C Scan
17:45:45.788 -> Found I2C Device at 0x11
17:45:45.788 -> Found I2C Device at 0x64
17:45:45.788 -> I2C Devices Found: 2
17:45:45.788 -> Publishing Message: I2CResponse...Published!
17:45:47.626 -> PING!
17:45:52.319 -> PING!
17:45:57.020 -> PING!
17:46:01.762 -> PING!
17:46:06.488 -> PING!
17:46:11.169 -> PING!
17:46:15.884 -> PING!
17:46:20.610 -> PING!
17:46:25.041 -> STATUS LED BLINK KAT
17:46:25.312 -> PING!
17:46:30.019 -> PING!

[brentru]

@tyeth Thank you for finding the bug. I assigned @lorennorman to take a look at it.

In the meantime, I am going to review this PR and merge it in. I'll also set the component to published. Likely a new WS release today, too, as I'm adding #411

[tyeth]

@brentru Thanks for the rapid work. Did you spot the packet size decode error returned? #408

[brentru]

I didn't, added to my assignments.