Calculate average baro reading and readjust offset on landing
Reset surface offs when landed, add 3 new inav params
Added altitude and baro offset correction using sonar.
Use PX4 board height from bottom of vehicle for starting altitude.
Reset Z velocity and surface rate while landed.
Added PX4FLOW and PX4 offset params to sonar correction.
Reset surface_offset_rate when using sonar
Use only baro_corr to correct baro offset when using sonar for alt.
Adjusted baro_offset correction and fixed baro alt correction.
Readjust corr_sonar even if we have no new readings.
Invert corr_sonar sign
Revert various sonar and baro related changes.
Fixed baro offset correction using sonar.
Use z offset of PX4FLOW for more accuracy, remove PX4_Z_OFF param.

Fixed the use of PX4Flow "height from ground when landed" param to further
improve the "distance from bottom" accuracy on low altitudes.
Added/clarified comments.
Fixed baro offset being reset
Merge branch 'sonar_alt' into iarc2014