- Added passive forces plugins

- Added new `cable` composite type:
  * The `initial` parameter specifies the joint at the starting boundary: `free`, `ball`, or `none`.
  * The boundary bodies are exposed with the names:`B_left` and `B_right`.
  * The vertex initial positions can be specified directly in the XML with the parameter `vertex`.
  * The orientation of the body frame **is** the orientation of the material frame of the curve.

- Added new `cable` passive force plugin:
  * Twist and bending stiffness can be set separately with the parameters `twist` and `bend`.
  * The stress-free configuration can be set to be the initial one or flat with the flag `flat`.
  * New cable example showing the formation of plectoneme.
  * New coil example.
  * New belt example showing interaction between twist and anisotropy.
  * Added test using cantilever exact solution.

PiperOrigin-RevId: 480033694
Change-Id: I491271bce8fccb185961477e903e5a72d172c8a3

src/user/user_model.cc

@@ -2601,7 +2601,14 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
       }
     }
 
-    // set sensor_plugin to the plugin instance ID
+    for (int i = 0; i < nbody; ++i) {
+      if (bodies[i]->is_plugin) {
+        m->body_plugin[i] = bodies[i]->plugin_instance->id;
+      } else {
+        m->body_plugin[i] = -1;
+      }
+    }
+
     std::vector<std::vector<int>> plugin_to_sensors(nplugin);
     for (int i = 0; i < nsensor; ++i) {
       if (sensors[i]->type == mjSENS_PLUGIN) {
@@ -2623,6 +2630,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
       }
       int nstate = plugin->nstate(m, i);
       m->plugin_stateadr[i] = stateadr;
+      m->plugin_statenum[i] = nstate;
       stateadr += nstate;
       if (plugin->type & mjPLUGIN_SENSOR) {
         for (int sensor_id : plugin_to_sensors[i]) {