[WIP] navigator initialization and mode change improvements

[dagar]

• safely initialize every MissionBlock's _mission_item
• allow position setpoint triplets with no valid position setpoint to be published
  • this ensures that the position controllers don't accidentally use an old position setpoint
• move loiter and takeoff reposition handling into each class (full position setpoint triplet isn't needed)
• delete dead code

Needs testing

Test

• takeoff -> hold
• reposition usage
• switching modes

[dakejahl]

SITL test's worked fine, real world test not so much....
https://review.px4.io/plot_app?log=f464696e-00bb-48a8-b82f-00e089d2fa98

1. Issued a takeoff
2. After successful takeoff then transition into loiter, changed to position
3. Flew it forward a bit, no issues
4. Issued return
5. Quad begin to roll to reposition...kept rolling and then went inverted and smashed into the ground

This is flying todays master with pr-navigator-robust pulled on top of that.

[dakejahl]

FYI @dagar I flew this again but this time using an RC setup.... Definitely not safe to fly! haha

https://review.px4.io/plot_app?log=0d5f3f53-d3cc-4946-b125-cfeb4881ac8a

edit: hardfaults as it starts to roll to move towards home...

Sanity check.... SITL works fines, no local changes in navigator

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Closing as stale.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[dakejahl]

💩

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[dagar]

The majority of the changes here eventually ended up in master.