Add position actuator's timeconst attribute to PyMJCF schema.

PiperOrigin-RevId: 635187874
Change-Id: I983b21497d7d464108851fc5aeafc2e37ea91444

Author: erez-tom

dm_control/mjcf/schema.xml

@@ -444,6 +444,7 @@
             <attribute name="user" type="array" array_type="float"/>
             <attribute name="kp" type="float"/>
             <attribute name="kv" type="float"/>
+            <attribute name="timeconst" type="float"/>
           </attributes>
         </element>
         <element name="velocity">
@@ -750,6 +751,7 @@
                 <attribute name="user" type="array" array_type="float"/>
                 <attribute name="kp" type="float"/>
                 <attribute name="kv" type="float"/>
+                <attribute name="timeconst" type="float"/>
               </attributes>
             </element>
             <element name="velocity">
@@ -1997,6 +1999,7 @@
             <attribute name="user" type="array" array_type="float"/>
             <attribute name="kp" type="float"/>
             <attribute name="kv" type="float"/>
+            <attribute name="timeconst" type="float"/>
           </attributes>
         </element>
         <element name="velocity" repeated="true" namespace="actuator">