position_estimator_inav: optical flow support

 - Implemented optical flow support
 - Sonar fixes, altitude and climb rate estimation with sonar should be much more accurate now
 - Accelerometer bias estimation using all available sources
 - Allow EASY mode with only velocity estimate (from FLOW), WARNING: seamless transition from FLOW to GPS not implemented yet

PX4FLOW firmware with disabled gyro compensation filtering should be used: https://github.com/DrTon/Flow/tree/simple_gyro_comp

Don't merge. WIP, not tested in flight yet