[Model] Introduce the generalized gravity vector

src/multibody/model.hpp

@@ -385,10 +385,15 @@ namespace se3
     /// \brief Vector of joint torques (dim model.nv).
     Eigen::VectorXd tau;
     
-    /// \brief Vector of Non Linear Effects (dim model.nv). It corresponds to the Coriolis, centrifugal and gravitational effects.
-    /// \note  In the equation \f$ M\ddot{q} + b = \tau \f$,
-    ///        the non linear effects are associated to the \f$b\f$ term.
+    /// \brief Vector of Non Linear Effects (dim model.nv). It corresponds to concatenation of the Coriolis, centrifugal and gravitational effects.
+    /// \note  In the multibody dynamics equation \f$ M\ddot{q} + b(q, \dot{q}) = \tau \f$,
+    ///        the non linear effects are associated to the term \f$b\f$.
     Eigen::VectorXd nle;
+    
+    /// \brief Vector of generalized gravity (dim model.nv).
+    /// \note  In the multibody dynamics equation \f$ M\ddot{q} + c(q, \dot{q}) + g(q) = \tau \f$,
+    ///        the gravity effect is associated to the \f$g\f$ term.
+    Eigen::VectorXd g;
 
     /// \brief Vector of absolute operationnel frame placements (wrt the world).
     container::aligned_vector<SE3> oMf;

src/multibody/model.hxx

@@ -242,6 +242,7 @@ namespace se3
     ,liMi((std::size_t)model.njoints)
     ,tau(model.nv)
     ,nle(model.nv)
+    ,g(model.nv)
     ,oMf((std::size_t)model.nframes)
     ,Ycrb((std::size_t)model.njoints)
     ,dYcrb((std::size_t)model.njoints)