Better use of the barometer

Hi Orux,

I'm using your app on a GNex with the barometer enabled and while it's ok I feel it could be a bit better in three ways (I only really care about point 1, but being there...).
1) there is no button to calibrate it, I need to go through menus. A button in the sidebars would be much more practical.
2) the altimeter isn't shown without the GPS on but it could be useful even when the GPS is off.
3) It seems to me that calibration on the going isn't used in the statistics. It would be cool if the track readings could be corrected proportionally. For example I start my hike at 1000m and end at 1500m, but the altimeters shows 1550m. When I calibrate it at the and I get a sudden jump in the graphs from 1550 to 1500. Also, stats are falsed. Ideally the data between 1000 and 1550 should be remapped between 1000 and 1500 in a continuous way. I tried to use DEM based correction afterwards, but I got a lot of spikes.