- better in terms of dependencies:
  - the position controller code depended on position topics for yaw
  - mc_pos_control does not have to be run for Stabilized mode
- the code path is much simpler, and thus less error prone. This is
  especially important since Stabilized is often used as a fail-safe flight
  mode.

use nullptr for instance to make sure we always get the first instance.

Previously when switching e.g. from stabilized from acro, the stabilized
flight task kept running and publishing setpoints.
Luckily it caused no problems, but the log showed arbitrary attitude
setpoints.

Revert "pwm_out_sim lazily publish actuator_outputs"

This reverts commit bcad940a.

As there is only one bit used in 'flags', and it is only used in case
update_interval is not null, we can move the bit to UpdateIntervalData.

The size of UpdateIntervalData does not increase (on 32 bit).

Reduces RAM usage by 3.6KB (tested on a Pixracer).

- move the update after the integration: a new computed jerk has an impact at the next epoch only
- add jerk reduction in case of too large integration time: when a jerk of "min_jerk" during dt is too much
- add jerk reduction if the integration time is larger than the predicted one and that integrating that jerk would lead to an acceleration overshoot
- rename some variables

Trajectory - Add time synchronization between trajectories. Split update(...) function into updateDurations(...) and integrate(...) to be able to insert time synchronization in between.

Derivation by Mathieu Bresciani:
https://github.com/Auterion/trajectory_generator

- do not set zero throttle during the entire back-transition because otherwise
we need to make the back-transition really short
- added ramping up of throttle setpoint during backtransition to avoid
step inputs
- back-transition ends after back-transition time and not when motors are fully
rotated updwards. previously the vehicle would enter hover mode at high speed
which was not handled well by the mc position controller

Signed-off-by: Roman <bapstroman@gmail.com>

* Add attitude sub to mc_pos_control for yaw usage.

* Initialize pos control _states struct.

* Remove unnecessary init for struct in mc pos ctrl.

* Only use att topic for yaw setting in FlightTask.

Signed-off-by: Roman <bapstroman@gmail.com>

Signed-off-by: Roman <bapstroman@gmail.com>

Signed-off-by: Roman <bapstroman@gmail.com>

- fsync can be a long blocking operation, so we need to make sure the main
  logger thread does not block during this time, when it tries to aquire
  the mutex
- fixes calling fsync on an invalid file descriptor

 - only used in test routines

 - only SENSOR_POLLRATE_DEFAULT is needed