Implement multi-cell finite element method for interpolated flexes.

This change introduces a `flex_cellcount` field to `mjModel` to specify the number of cells in each dimension for interpolated flexes. The stiffness computation, passive force calculation, and Jacobian derivatives are updated to operate on a per-cell basis, significantly improving performance by localizing computations to the nodes within each cell.

PiperOrigin-RevId: 901216393
Change-Id: Ic23132e609de11e71bb7fef8d1f139daad2ec264

src/user/user_flexcomp.h

@@ -78,6 +78,7 @@ class mjCFlexcomp {
   std::string name;               // flex name
   mjtFcompType type;              // flexcomp type
   int count[3];                   // grid count in each dimension
+  int cellcount[3];               // number of cells for interpolation
   double spacing[3];              // spacing between grid elements
   double scale[3];                // scaling for mesh and direct
   double origin[3];               // origin for generating a 3D mesh from a convex 2D mesh