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/ekf2/ekf2_params.c

@@ -820,19 +820,6 @@ PARAM_DEFINE_FLOAT(EKF2_TERR_NOISE, 5.0f);
  */
 PARAM_DEFINE_FLOAT(EKF2_TERR_GRAD, 0.5f);
 
-/**
- * Device id of IMU
- *
- * Set to 0 to use system selected (sensor_combined) IMU,
- * otherwise set to the device id of the desired IMU (vehicle_imu).
- *
- * @group EKF2
- * @value 0 System Primary
- * @category Developer
- *
- */
-PARAM_DEFINE_INT32(EKF2_IMU_ID, 0);
-
 /**
  * X position of IMU in body frame (forward axis with origin relative to vehicle centre of gravity)
  *