- 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

include/mujoco/mjplugin.h

@@ -20,7 +20,8 @@
 
 typedef enum mjtPluginTypeBit_ {
   mjPLUGIN_ACTUATOR = 1<<0,
-  mjPLUGIN_SENSOR   = 1<<1
+  mjPLUGIN_SENSOR   = 1<<1,
+  mjPLUGIN_PASSIVE  = 1<<2,
 } mjtPluginTypeBit;
 
 struct mjpPlugin_ {
@@ -38,8 +39,8 @@ struct mjpPlugin_ {
   // dimension of the specified sensor's output (required only for sensor plugins)
   int (*nsensordata)(const mjModel* m, int instance, int sensor_id);
 
-  // called when a new mjData is being created (required)
-  void (*init)(const mjModel* m, mjData* d, int instance);
+  // called when a new mjData is being created (required), returns 0 on success or -1 on failure
+  int (*init)(const mjModel* m, mjData* d, int instance);
 
   // called when an mjData is being freed (optional)
   void (*destroy)(mjData* d, int instance);