Add two new attributes to weld constraints:

- `anchor` determines the point of wrench application, in the frame of body2.
- `tfratio` scales applied torques relative to applied forces.
- Add visualisation of both anchor points to both weld and connect constraints.
- Add a test model showing how the new weld parameters behave.

PiperOrigin-RevId: 469228149
Change-Id: I2863b8a52b35bc562d37fd2d779dd8b8f0bea24a

Author: DeepMind

dm_control/mjcf/schema.xml

@@ -1398,8 +1398,6 @@
             <attribute name="body1" type="reference" required="true" reference_namespace="body"/>
             <attribute name="body2" type="reference" reference_namespace="body"/>
             <attribute name="relpose" type="array" array_type="float" array_size="7"/>
-            <attribute name="anchor" type="array" array_type="float" array_size="3"/>
-            <attribute name="tfratio" type="float"/>
           </attributes>
         </element>
         <element name="joint" repeated="true" namespace="equality">