Create interface for SHT31 custom calibration

[oscgonfer]

This adds an interface to calibrate temperature and humidity sensor offsets:

control temp cal

control hum cal

This will offer an interface for people to add an offset. This offset will be added to predefined ones, already tested on each hardware version, but that may require tweaks based on the enclosure, deployment conditions, etc.

[oscgonfer]

Example:

SCK > control temp
Temperature: 
Available commands:
* cal: [float - clear] Sets temperature offset with respect to current offset or clears it
SCK > control temp cal 
Temperature: cal
Current temperature offset: -1.00 (degC)
SCK > control temp cal -10
Temperature: cal -10
SCK > read temp
Temperature: 17.35 C
SCK > control temp cal clear
Temperature: cal clear
Set default offset: -1.00 (degC)
SCK > read temp
Temperature: 27.37 C