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: I219ff09197ba8ecb5cbf1ebfc0b00539718a33fd

Author: quagla

dm_control/mjcf/schema.xml

@@ -1202,6 +1202,7 @@
             <attribute name="group" type="int"/>
             <attribute name="type" type="keyword" valid_values="grid box cylinder ellipsoid mesh gmsh direct"/>
             <attribute name="count" type="array" array_type="int" array_size="3"/>
+            <attribute name="cellcount" type="array" array_type="int" array_size="3"/>
             <attribute name="spacing" type="array" array_type="float" array_size="3"/>
             <attribute name="scale" type="array" array_type="float" array_size="3"/>
             <attribute name="radius" type="float"/>
@@ -1771,6 +1772,8 @@
             <attribute name="texcoord" type="array" array_type="float"/>
             <attribute name="elemtexcoord" type="array" array_type="float"/>
             <attribute name="node" type="array" array_type="float" array_size="3"/>
+            <attribute name="cellcount" type="array" array_type="int" array_size="3"/>
+            <attribute name="dof" type="keyword" valid_values="trilinear quadratic"/>
           </attributes>
           <children>
             <element name="edge" repeated="true">