- 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/xml/xml_native_writer.cc

@@ -1373,6 +1373,28 @@ void mjXWriter::Body(XMLElement* elem, mjCBody* body) {
     OneLight(InsertEnd(elem, "light"), body->lights[i], body->lights[i]->def);
   }
 
+  // write plugin
+  if (body->is_plugin) {
+    XMLElement *child = InsertEnd(elem, "plugin");
+    if (!body->plugin_instance_name.empty()) {
+      WriteAttrTxt(child, "instance", body->plugin_instance_name);
+    } else {
+      WriteAttrTxt(child, "plugin", body->plugin_name);
+      const mjpPlugin* plugin = mjp_getPluginAtSlot(
+          body->plugin_instance->plugin_slot);
+      const char* c = &body->plugin_instance->flattened_attributes[0];
+      for (int i = 0; i < plugin->nattribute; ++i) {
+        std::string value(c);
+        if (!value.empty()) {
+          WriteAttrTxt(child, std::string("plugin:") + plugin->attributes[i],
+                        value);
+          c += value.size();
+        }
+        ++c;
+      }
+    }
+  }
+
   // write child bodies recursively
   for (i=0; i<body->bodies.size(); i++) {
     Body(InsertEnd(elem, "body"), body->bodies[i]);