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/uuv_att_control/uuv_att_control.cpp

@@ -59,7 +59,7 @@ extern "C" __EXPORT int uuv_att_control_main(int argc, char *argv[]);
 
 UUVAttitudeControl::UUVAttitudeControl():
 	ModuleParams(nullptr),
-	WorkItem(MODULE_NAME, px4::wq_configurations::attitude_ctrl),
+	WorkItem(MODULE_NAME, px4::wq_configurations::nav_and_controllers),
 	/* performance counters */
 	_loop_perf(perf_alloc(PC_ELAPSED, MODULE_NAME": cycle"))
 {