Changes to inertia inference from meshes.

PiperOrigin-RevId: 726051033
Change-Id: I7fb72d65f602815539c0e379ae3ae76f15738f0d

Author: powertj

dm_control/mjcf/schema.xml

@@ -233,7 +233,7 @@
           <attributes>
             <attribute name="scale" type="array" array_type="float" array_size="3"/>
             <attribute name="maxhullvert" type="int"/>
-            <attribute name="inertia" type="keyword" valid_values="legacy convex exact"/>
+            <attribute name="inertia" type="keyword" valid_values="legacy convex exact shell"/>
           </attributes>
         </element>
         <element name="material">