Add quadratic interpolation to flex.

PiperOrigin-RevId: 822055781
Change-Id: Ibc252ac560de666b0b28f868e46b88d3ed92f67d

src/user/user_objects.h

@@ -1025,6 +1025,8 @@ class mjCFlex: public mjCFlex_, private mjsFlex {
 
   static constexpr int kNumEdges[3] = {1, 3, 6};  // number of edges per element indexed by dim
 
+  void SetOrder(int order) { order_ = order; }  // set interpolation order
+
  private:
   void Compile(const mjVFS* vfs);         // compiler
   void CreateBVH(void);                   // create flex BVH
@@ -1032,6 +1034,8 @@ class mjCFlex: public mjCFlex_, private mjsFlex {
 
   std::vector<double> vert0_;             // vertex positions in [0, 1]^d in the bounding box
   std::vector<double> node0_;             // node Cartesian positions
+
+  int order_ = 0;                         // interpolation order
 };