MakeParticle and elasticity plugin refactoring.

- Removed the explicit dependency on a plugin name.
- Creation of particles from user-defined vertices (if a plugin is present).
- Add skin support for an irregular mesh
- Fixes #642 and #674.

PiperOrigin-RevId: 561313771
Change-Id: I450f91f0d3b578cc18d97ed7180a0a8944b7dd41

plugin/elasticity/solid.h

@@ -63,10 +63,10 @@ class Solid {
   mjtNum damping;
 
  private:
-  Solid(const mjModel* m, mjData* d, int instance, int nx, int ny, int nz,
-        mjtNum nu, mjtNum E, mjtNum damp);
+  Solid(const mjModel* m, mjData* d, int instance, mjtNum nu, mjtNum E,
+        mjtNum damp, const std::vector<int>& simplex);
 
-  void CreateStencils(int nx, int ny, int nz);
+  void CreateStencils(const std::vector<int>& simplex);
 };
 
 }  // namespace mujoco::plugin::elasticity