Introduces a new "strain" equality mode (flex_edgeequality = 3) for flexcomps with trilinear interpolation. This mode enforces constraints on the three invariants of the Green-Lagrange strain tensor at 8 Gauss quadrature points within each flex element, resulting in 24 equality constraints per flexcomp.

Also fixes a bug when attaching a flex with constraints.

PiperOrigin-RevId: 881349765
Change-Id: I0561aa4f62442a8ba53c96b8328110c8a27cc8a0

Author: quagla

dm_control/mjcf/schema.xml

@@ -1909,6 +1909,16 @@
             <attribute name="flex" type="identifier"/>
           </attributes>
         </element>
+        <element name="flexstrain" repeated="true" namespace="equality">
+          <attributes>
+            <attribute name="name" type="identifier"/>
+            <attribute name="class" type="reference" reference_namespace="default"/>
+            <attribute name="active" type="keyword" valid_values="false true"/>
+            <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"/>
+          </attributes>
+        </element>
       </children>
     </element>
     <element name="tendon">