Add per-body gravity compensation (buoyancy) passive force.

PiperOrigin-RevId: 485342531
Change-Id: Icb3e6bb5080b6ef17a3d3b1cc67e0503cfeaeffb

src/user/user_model.cc

@@ -1285,6 +1285,7 @@ void mjCModel::CopyTree(mjModel* m) {
     copyvec(m->body_iquat+4*i, pb->lociquat, 4);
     m->body_mass[i] = (mjtNum)pb->mass;
     copyvec(m->body_inertia+3*i, pb->inertia, 3);
+    m->body_gravcomp[i] = pb->gravcomp;
     copyvec(m->body_user+nuser_body*i, pb->userdata.data(), nuser_body);
 
     // count free joints

src/user/user_objects.cc

@@ -326,6 +326,7 @@ mjCBody::mjCBody(mjCModel* _model) {
   weldid = -1;
   dofnum = 0;
   lastdof = -1;
+  gravcomp = 0;
   userdata.clear();
 
   // plugin variables

src/user/user_objects.h

@@ -186,6 +186,7 @@ class mjCBody : public mjCBase {
   double iquat[4];                // inertial frame orientation
   double mass;                    // mass
   double inertia[3];              // diagonal inertia (in i-frame)
+  double gravcomp;                // gravity compensation
   std::vector<double> userdata;   // user data
   mjCAlternative alt;             // alternative orientation specification
   mjCAlternative ialt;            // alternative for inertial frame