This change lead to a build error and is therefore reverted here.

    - matrix in PX4/Firmware (a138252aea03ff414ef5b4444c61e2f7d5075ebc): https://github.com/PX4/Matrix/commit/21d47424c6050bb94da5de3f7580a8df66b6fcc7
    - matrix current upstream: https://github.com/PX4/Matrix/commit/03a3e3ad46d21bb90e78f8cedd3526d24617df5f
    - Changes: https://github.com/PX4/Matrix/compare/21d47424c6050bb94da5de3f7580a8df66b6fcc7...03a3e3ad46d21bb90e78f8cedd3526d24617df5f

    03a3e3a 2018-06-09 Daniel Agar - helper_functions add wrap_2pi
abc8f82 2018-06-09 Daniel Agar - travis-ci add codecov.io (#69)

analyse_logdata_ekf:
- add sideslip innovation fail check flag to fix wrongly selected
bits for hagl innovation and optical flow innovations
- plot sideslip innovation fail
- add gps fix type fail flag to fix wrongly selected bits for all
gps check fail flags
- plot gps fix type

    - sitl_gazebo in PX4/Firmware (1b33445c): https://github.com/PX4/sitl_gazebo/commit/e8b3624b9dad63a492ae58c1ce8661488128966b
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/59f01ae9bd8ee1937be368f9b536fda02bf1aba6
    - Changes: https://github.com/PX4/sitl_gazebo/compare/e8b3624b9dad63a492ae58c1ce8661488128966b...59f01ae9bd8ee1937be368f9b536fda02bf1aba6

    59f01ae 2018-06-07 Julian Oes - cmake: add tinyxml dependency

    - ecl in PX4/Firmware (f7937d783496e954efc52439148ef66824d9c80a): https://github.com/PX4/ecl/commit/1fdf33b343e361de6410515a0359f3cb7f34d499
    - ecl current upstream: https://github.com/PX4/ecl/commit/d177e96508d2572f6fa8eb7ff41852749c882548
    - Changes: https://github.com/PX4/ecl/compare/1fdf33b343e361de6410515a0359f3cb7f34d499...d177e96508d2572f6fa8eb7ff41852749c882548

    d177e96 2018-06-08 Paul Riseborough - EKF: Fix bug causing slow drift when high rate flow data is used
ee2dc7d 2018-05-30 Paul Riseborough - EKF: Rework optical flow selection logic
e383b6a 2018-05-29 Paul Riseborough - EKF: rework optical flow selection logic
487e6a0 2018-05-28 Paul Riseborough - EKF: enable user selection of auto mag free operation
6bdbe03 2018-05-28 Paul Riseborough - EKF: Fallback to optical flow for all in-flight loss of navigation scenarios
b4d2b8c 2018-05-19 Mohammed Kabir - EKF : introduce new architechture for navigation limits
8a71339 2018-05-19 Paul Riseborough - EKF: Improve ground effect compensation
39697f1 2018-05-18 Paul Riseborough - EKF: rework optical flow switching
1cfe845 2018-05-18 Paul Riseborough - EKF: rework GPS quality check
99a8038 2018-05-18 Paul Riseborough - EKF: improve optical flow GPS quality checking
7f36add 2018-05-18 Paul Riseborough - EKF: scale GPS vertical accuracy check when using optical flow
fc9f532 2018-05-18 Paul Riseborough - EKF: relax range finder data continuity check
93c456f 2018-05-18 Paul Riseborough - EKF: Improve protection against badly conditioned dVel bias covariances
225057a 2018-05-18 Paul Riseborough - EKF: Fix bug preventing use of terrain estimator
adb4a09 2018-05-17 Paul Riseborough - EKF: Fix bug causing large yaw innovations when GPS is lost
f59cd0f 2018-05-16 Paul Riseborough - EKF: Don't make detection of indoor operation dependent on optical flow
1562a82 2018-05-16 Paul Riseborough - EKF: Add parameter to adjust on-ground movement detector sensitivity
ea9e824 2018-05-16 Paul Riseborough - EKF: Improve detection of indoor flight condition
565f992 2018-05-16 Paul Riseborough - EKF: Reduce effect of yaw gyro bias when using optical flow indoors
e10798b 2018-05-16 Paul Riseborough - EKF: Add on ground movement detector
2d3b652 2018-05-15 Paul Riseborough - EKF: Reset yaw gyro bias learning when resuming use of magnetometer
8191068 2018-05-15 Paul Riseborough - EKF: Don't start using mag if optical flow use is interrupted
4889e84 2018-05-15 Paul Riseborough - EKF: Don't fuse multi rotor drag if yaw angle is bad
092a8d8 2018-05-15 Paul Riseborough - EKF: Fix GPS validity time check error
0160aaa 2018-05-15 Paul Riseborough - EKF: Don't use magnetometer with optical flow only nav if GPS checks are failing
8451676 2018-05-14 Paul Riseborough - EKF: Use stricter GPS accuracy test when optical flow is being used
a80b3ab 2018-05-27 Daniel Agar - set MODULE define for each library

- P and D gains are too high for a racer
- default I gain is too low (0.25 is still quite low)
- use the thrust curve param instead of TPA
- improve responsiveness in Manual & increase max tilt angle to 60 degrees
- enable one-shot
- enable high-rate logging profile
- disable RC filter

 - remove QURT defines no longer required

Implements a better method of determining when to switch from velocity to altitude control to keep height limit from being exceeded.
This method removes the overshoot and transients in height caused by the switching of the previous algorithm.

The correction for stopping distance applied to the maximum altitude limiter uses the vertical velocity estimate and gives the same offset for both positive (down) and negative (up) velocity.
This calculation has been corrected and simplified and variable names changes to make the functionality clearer.

Fixes a race condition introduced by use of _is_dead-reckoning.
Only runs flow use logic when there is flow data available