This allows us to detect if the I2C device is indeed an Si7021.
By reading status, turning the heater on, reading status, and turning
the heater off, reading status, we will see three different states:
status bit13 is 'heater on' (0=off, 1=on).
In status0, bit13 will be 0,
In status1, bit13 will be 1, (the heater is now on)
In status2, bit13 will be 0.