Imu improvements

Write tests for IMU, pass them.

This MR has little to contribute to Master apart from IMU, but there are a few little impros.

src/processor_imu.h

@@ -72,6 +72,7 @@ class ProcessorIMU : public ProcessorMotion{
                                                const MatrixXs& _data_cov,
                                                const FrameBasePtr& _frame_origin) override;
         virtual FeatureBasePtr createFeature(CaptureMotionPtr _capture_motion) override;
+    public:
         virtual ConstraintBasePtr emplaceConstraint(FeatureBasePtr _feature_motion,
                                                     CaptureBasePtr _capture_origin) override;