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_init.c

@@ -224,6 +224,9 @@ void mjs_defaultFlex(mjsFlex* flex) {
   // set other defaults
   flex->dim = 2;
   flex->radius = 0.005;
+  flex->cellcount[0] = 1;
+  flex->cellcount[1] = 1;
+  flex->cellcount[2] = 1;
   flex->internal = 0;
   flex->selfcollide = mjFLEXSELF_AUTO;
   flex->activelayers = 1;