meas_airspeed: don't take the aboslute value
airspeed_calibration: stop talking about Pa and and hashtags
Revert "airspeed_calibration: stop talking about Pa and and hashtags"

This reverts commit c6fb75f66fa94444d07056f271980e3f05008f94.
airspeed_calibration: stop talking about Pa and and hashtags (now the correct files)
meas_airspeed: don't reset the filter below 0
Remove absolute pressure field as its not useful and confusing anywary
Merge pull request #1308 from PX4/airspeedfabs

No absolute value of airspeed