Add a number of missing struct fields to Python bindings.

It is quite possible that there are a few other fields still missing since we currently do not yet have automated testing of struct field existence.

Fixes #747.

PiperOrigin-RevId: 517189048
Change-Id: I7c1d95e700eaea05aaa3cde1fd05e1a259553117

python/mujoco/structs.h

@@ -611,6 +611,7 @@ class MjWrapper<raw::MjvPerturb> : public WrapperBase<raw::MjvPerturb> {
   #define X(var) py_array_or_tuple_t<mjtNum> var
   X(refpos);
   X(refquat);
+  X(refselpos);
   X(localpos);
   #undef X
 };