Add membrane plugin (2D Flex elastic stiffness).

PiperOrigin-RevId: 574109411
Change-Id: I4a701d8189cecf540cd200bd30cb4582f3c7dd43

plugin/elasticity/solid.h

@@ -21,17 +21,11 @@
 #include <mujoco/mjdata.h>
 #include <mujoco/mjmodel.h>
 #include <mujoco/mjtnum.h>
+#include "elasticity.h"
 
 
 namespace mujoco::plugin::elasticity {
 
-struct Stencil3D {
-  static constexpr int kNumEdges = 6;
-  static constexpr int kNumVerts = 4;
-  int vertices[kNumVerts];
-  int edges[kNumEdges];
-};
-
 class Solid {
  public:
   // Returns a new Solid instance or nullopt on failure.
@@ -67,9 +61,6 @@ class Solid {
   Solid(const mjModel* m, mjData* d, int instance, mjtNum nu, mjtNum E,
         mjtNum damp, const std::vector<int>& simplex,
         const std::vector<int>& edgeidx);
-
-  void CreateStencils(const std::vector<int>& simplex,
-                      const std::vector<int>& edgeidx);
 };
 
 }  // namespace mujoco::plugin::elasticity