Add clock sensor type.

PiperOrigin-RevId: 459720950
Change-Id: Ibda48ae7dc4aa7be451a947a1c53d10a7eaf176b

src/user/user_objects.cc

@@ -3533,7 +3533,7 @@ void mjCSensor::Compile(void) {
 
     // get sensorized object id
     objid = pobj->id;
-  } else {
+  } else if (type != mjSENS_CLOCK) {
     throw mjCError(this, "invalid type in sensor '%s' (id = %d)", name.c_str(), id);
   }
 
@@ -3804,6 +3804,12 @@ void mjCSensor::Compile(void) {
     }
     break;
 
+  case mjSENS_CLOCK:
+    dim = 1;
+    needstage = mjSTAGE_POS;
+    datatype = mjDATATYPE_REAL;
+    break;
+
   case mjSENS_USER:
     // check for negative dim
     if (dim<0) {