Resolve "Adapt to const nonconst new API"

Closes #25 (closed)

include/imu/factor/factor_imu.h

@@ -478,8 +478,8 @@ inline bool FactorImu::residual(const Eigen::MatrixBase<D1> &       _p1,
 
 inline Eigen::VectorXd FactorImu::expectation() const
 {
-    FrameBasePtr frm_current = getFeature()->getFrame();
-    FrameBasePtr frm_previous = getFrameOther();
+    auto frm_current = getFeature()->getFrame();
+    auto frm_previous = getFrameOther();
 
     //get information on current_frame in the FactorImu
     const Eigen::Vector3d frame_current_pos    = (frm_current->getP()->getState());