Temperature/Humidity and Formaldehyde calculation are wrong for 5003ST

[fmatray]

Hello, I just test the library with a PMS5003ST and those values were wrong :

• temperature was - 5.5°C instead of something around 22°
• Humidity was also bad.

The problems were :

• int8_t cast instead of int16_t
• incorrect reading order. The good one, according to the datasheet, is : hcho, temp, rhum.

I post a PR to correct theses points.

I noticed a difference between the sensor and a regular thermometer. So I also add a feature to add offset to temperature and humidity to correct theses values :

• void set_rhum_offset(float offset);
• void set_temp_offset(float offset);
• float get_rhum_offset();
• float get_temp_offset();

This is based on the library GHTNew (https://github.com/RobTillaart/DHTNew).