Refactor flex strain constraints to be per-cell.

Each mjEQ_FLEXSTRAIN equality now represents a single cell within a flex. This allows for more efficient sparse Jacobian computation by only considering the degrees of freedom of the nodes within each specific cell. This change gives a speedup of about 10x on a 3x3x3 model.

PiperOrigin-RevId: 902164069
Change-Id: I3fefdaceedeac1e1232672f2b76e954f2d027b2f

Author: quagla

dm_control/mjcf/schema.xml

@@ -1947,6 +1947,8 @@
             <attribute name="solref" type="array" array_type="float" array_size="2"/>
             <attribute name="solimp" type="array" array_type="float" array_size="5"/>
             <attribute name="flex" type="identifier"/>
+            <attribute name="cell" type="array" array_type="int" array_size="3"/>
+
           </attributes>
         </element>
       </children>