Orbit approach transition improvements

[MaEtUgR]

Describe problem solved by this pull request
It's possible to have steps when entering and exiting the circle approach:

Describe your solution

1. @ThomasDebrunner removed switching to a circle approach if the radius is just slightly adjusted using the stick.
2. I found a typo and did some variable name improvement refactoring.
3. I checked how the position smoothing is initialized and made sure it's using the previous setpoint rather than the current vehicle state. This should give better results because the resulting error should then be continuous while if the setpoint jumps to the current vehicle state the error jumps to zero.

Describe possible alternatives
We should further improve the interface for using the jerk trajectory library to make it easier to reuse.

Test data / coverage
So far only parts are tested. I still have to test the latest state of this which I'll do the day after tomorrow.

[ThomasDebrunner]

Thanks for cleaning up, looks good to me!

[MaEtUgR]

Something is sadly broken in the reapproach 😞 I'll fix it and report back.

[MaEtUgR]

Took much longer to figure all the problems out but now it seems ok to me but my learning is that we can't take care of all smooth transitions individually there are too many errors to be made. I suggest we try to have a jerk trajectory generator running more or less all the time and adjust the parameters or skip it only for when it's necessary.

Problems were:

• Invalid setpoints could lead to flyaway: Multicopter position control: Invalid setpoint can windup integrator #19041
• There were invalid altitude acceleration setpoints because I initialized the altitude smoothing for the next circle approach with the altitude setpoints from the circle smoothing but acceleration was not properly set.
• In the circle the stick-based smoothed altitude change did not work properly, setpoints like the acceleration could get stuck, there were jumps. I used the inheritance from FlightTaskManualAltitudeSmoothVel again to solve this and made sure the altitude change command works.
• In the vertical circle approach it would always use maximum downwards speed also for upwards.

The only remaining thing I could find is that the altitude given in an orbit command is not exactly reached because when close enough it switches back to stick altitude steering and to get to the exact commanded altitude smoothly is currently not possible without changing everything. This can be improved once we can easily give up the dependency on FlightTaskManualAltitudeSmoothVel.

[ThomasDebrunner]

Tested in SITL, all working fine. As a further improvement, we could limit the radial speed on orbit radius change

[MaEtUgR]

As a further improvement, we could limit the radial speed on orbit radius change

Like discussed I just added this (after a rebase on master) in 75445b4

	Before	After
Velocity for radius change	Maximum radius / 8 = 12m/s	Cruise speed default = 5m/s
Acceleration for velocity change	Maximum velocity / 4 = 2.5m/s^2	Manual horizontal acceleration default = 3m/s^2

[hamishwillee]

@MaEtUgR Are there any docs updates required for this in http://docs.px4.io/master/en/flight_modes/orbit.html#orbit-mc ?

(and the other PRS in PX4 1.13 release notes?

Specifically, I don't think we need to talk about improvements in the experience unless they have associated parameters that need to be changed, or if they change anything we say in the above doc.