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

test/engine/engine_forward_test.cc

@@ -3107,7 +3107,7 @@ TEST_F(ForwardTest, FlexParentCoupling) {
       <body name="parent" pos="0 0 0">
         <freejoint/>
         <geom size=".1" mass="0.1"/>
-        <flexcomp name="flex" type="grid" count="3 3 3" spacing="1 1 1"
+        <flexcomp name="flex" type="grid" count="3 3 3" cellcount="1 1 1" spacing="1 1 1"
                   radius=".01" dim="3" mass="100" dof="trilinear" pos="1 1 1">
           <contact selfcollide="none"/>
           <elasticity young="1e4" poisson="0.3" damping="50"/>
@@ -3152,7 +3152,7 @@ TEST_F(ForwardTest, FlexParentCoupling) {
     if (diff > max_diff) max_diff = diff;
   }
 
-  EXPECT_LT(max_diff, MjTol(2e-5, 5e-3))
+  EXPECT_LT(max_diff, MjTol(2e-5, 1.5e-2))
       << "Implicit integrator should match Euler at small timestep";
 
   mj_deleteData(data);