- 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/engine/engine_core_smooth.c

@@ -23,6 +23,7 @@
 #include "engine/engine_core_constraint.h"
 #include "engine/engine_io.h"
 #include "engine/engine_macro.h"
+#include "engine/engine_plugin.h"
 #include "engine/engine_support.h"
 #include "engine/engine_util_blas.h"
 #include "engine/engine_util_errmem.h"
@@ -1422,6 +1423,25 @@ void mj_passive(const mjModel* m, mjData* d) {
   if (mjcb_passive) {
     mjcb_passive(m, d);
   }
+
+  // plugin
+  if (m->nplugin) {
+    const int nslot = mjp_pluginCount();
+    // iterate over plugins, call compute if type is mjPLUGIN_PASSIVE
+    for (int i=0; i<m->nplugin; i++) {
+      const int slot = m->plugin[i];
+      const mjpPlugin* plugin = mjp_getPluginAtSlotUnsafe(slot, nslot);
+      if (!plugin) {
+        mju_error_i("invalid plugin slot: %d", slot);
+      }
+      if (plugin->type & mjPLUGIN_PASSIVE) {
+        if (!plugin->compute) {
+          mju_error_i("`compute` is a null function pointer for plugin at slot %d", slot);
+        }
+        plugin->compute(m, d, i, mjPLUGIN_PASSIVE);
+      }
+    }
+  }
 }