- 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.h

@@ -118,6 +118,7 @@ class mjCAlternative {
 //------------------------- class mjCBase ----------------------------------------------------------
 // Generic functionality for all derived classes
 
+class mjCPlugin;
 class mjCBase {
   friend class mjCDef;
 
@@ -127,10 +128,16 @@ class mjCBase {
   int id;                         // object id
   int xmlpos[2];                  // row and column in xml file
   mjCDef* def;                    // defaults class used to init this object
+  mjCModel* model;                // pointer to model that created object
+
+  // plugin support
+  bool is_plugin;
+  std::string plugin_name;
+  std::string plugin_instance_name;
+  mjCPlugin* plugin_instance;
 
  protected:
   mjCBase();                      // constructor
-  mjCModel* model;                // pointer to model that created object
 };