- 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_objects.cc

@@ -328,6 +328,12 @@ mjCBody::mjCBody(mjCModel* _model) {
   lastdof = -1;
   userdata.clear();
 
+  // plugin variables
+  is_plugin = false;
+  plugin_instance = nullptr;
+  plugin_name = "";
+  plugin_instance_name = "";
+
   // clear object lists
   bodies.clear();
   geoms.clear();
@@ -799,6 +805,21 @@ void mjCBody::Compile(void) {
 
   // compile all lights
   for (i=0; i<lights.size(); i++) lights[i]->Compile();
+
+  // plugin
+  if (is_plugin) {
+    if (plugin_name.empty() && plugin_instance_name.empty()) {
+      throw mjCError(
+          this, "neither 'plugin' nor 'instance' is specified for body '%s', (id = %d)",
+          name.c_str(), id);
+    }
+
+    model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
+    const mjpPlugin* plugin = mjp_getPluginAtSlot(plugin_instance->plugin_slot);
+    if (!(plugin->type & mjPLUGIN_PASSIVE)) {
+      throw mjCError(this, "plugin '%s' does not support passive forces", plugin->name);
+    }
+  }
 }