AP_NavEKF: report active EKF source sets and lanes

[landswellsong]

Adds the indication of currently active EKF source set (and core/lane for good measure). 1-based indices because it's how the source switch message was coded. I'm not entirely sure about whether to keep it or to change the switch message to 0-based scheme, both have pros and cons, can do as the reviewer decides.

mavlink PR: ArduPilot/mavlink#428

Works for me in SITL. Marking as WIP to get comments about the index domain.

CC @peterbarker

[peterbarker]

You can't assume extensions are passed as anything but zero, so using 1-based indexing is good.

[peterbarker]

I've just realised that this is somewhat in opposition to an alternate proposal I had many years ago allowing for each EKF lane (and, indeed, each estimator on the system) to emit EKF_STATUS_REPORT.

I don't think that should block this PR, but is information.

[landswellsong]

@peterbarker I kind of assumed it was already the case before I even started on this 😓 I think the approaches aren't mutually exclusive, if you want all the lanes to emit a packet you still want to tell the lanes apart. Sounds like an idea for iterating on this, but as to other estimators I am not sure. The packet already is EKF inspired so we pass a bunch of zeroes for DCM and external nav for example. Maybe it can be reworked a bit? (In future ofc)

[landswellsong]

As per devcall, added an estimator type enum as well as a few tweaks. I realized that DCM actually does not send EKF_STATUS_REPORT (which is reasonable), but it's SIM and EAHRS that do (which is less so). I therefore have set up the enum in such a way:

• 0 ⇒ External AHRS
• n ⇒ NavEKFn
• 255 ⇒ SIM

Please review and tell if this is fine (or should we maybe include DCM? to know that the system has degraded from EKF3 to DCM for example).

I also reordered the attributes a bit so that we can safely send zeroes for cores/souce sets EAHRS (if it has cores/source sets it shouldn't not be our concern and we probably don't have that info) and for the source sets for EKF2 which didn't have them back then.