Multi-EKF support (ekf2)

- ekf2 can now run in multi-instance mode (currently up to 9 instances)
    - in multi mode all estimates are published to alternate topics (eg estimator_attitude instead of vehicle_attitude)
 - new ekf2 selector runs in multi-instance mode to monitor and compare all instances, selecting a primary (eg N x estimator_attitude => vehicle_attitude)
 - sensors module accel & gyro inconsistency checks are now relative to the mean of all instances, rather than the current primary (when active ekf2 selector is responsible for choosing primary accel & gyro)
 - existing consumers of estimator_status must check estimator_selector_status to select current primary instance status
 - ekf2 single instance mode is still fully supported and the default

Co-authored-by: Paul Riseborough <gncsolns@gmail.com>

src/modules/sensors/sensors.cpp

@@ -536,7 +536,8 @@ void Sensors::InitializeVehicleIMU()
 			if (accel.device_id > 0 && gyro.device_id > 0) {
 				// if the sensors module is responsible for voting (SENS_IMU_MODE 1) then run every VehicleIMU in the same WQ
 				//   otherwise each VehicleIMU runs in a corresponding INSx WQ
-				const px4::wq_config_t &wq_config = px4::wq_configurations::nav_and_controllers;
+				const bool multi_mode = (_param_sens_imu_mode.get() == 0);
+				const px4::wq_config_t &wq_config = multi_mode ? px4::ins_instance_to_wq(i) : px4::wq_configurations::INS0;
 
 				VehicleIMU *imu = new VehicleIMU(i, i, i, wq_config);