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>

msg/estimator_selector_status.msg

@@ -0,0 +1,22 @@
+uint64 timestamp		# time since system start (microseconds)
+
+uint8 primary_instance
+
+uint8 instances_available
+
+uint32 instance_changed_count
+uint64 last_instance_change
+
+uint32 accel_device_id
+uint32 baro_device_id
+uint32 gyro_device_id
+uint32 mag_device_id
+
+float32[9] combined_test_ratio
+float32[9] relative_test_ratio
+bool[9] healthy
+
+float32[4] accumulated_gyro_error
+float32[4] accumulated_accel_error
+bool gyro_fault_detected
+bool accel_fault_detected