Altitude measurement

After the new offset configuration options are implemented, it鈥檇 make sense to instruct user on how to measure altitude changes using air pressure sensor.

User would reset first altitude offset to zero and then it鈥檇 be possible to measure altitude changes (in metres of feet) with the sensor.

This was originally an issue in consideration Feature suggestion: Altitude measurement 路 Issue #280 路 ruuvi/com.ruuvi.station 路 GitHub

Reminding user of original 鈥渓ocation鈥 and time seems awkward (?) .

How long would this be valid before atmospheric changes invalidate the reading?