run multiple AHRS/INS estimators in parallel

[flixr]

The goal is to have the possibility to run multiple state estimation algorithms at the same time and switch the output (to the state interface) between then during runtime.

Basic switching to a secondary AHRS is already possible with #1130. Moved the remaining TODO list to here.

Open Questions/TODO:

• [ ] implement switching for INS
  • [ ] deal with full INS (e.g. invariant) that also outputs attitude
• [ ] deal with the remaining "global" ins_reset_x functions (ins_reset_local_origin, ins_reset_altitude_ref, ...)
• [x] get rid of ins_periodic for those impls which need it (probably when we start converting them to modules)
• [x] deal with multiple implementations registering the same telemetry message #1440 #1441

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[gautierhattenberger]

Not closing this for now as a reminder, however the proper way to handle this needs to be clarified.