Replace flex_xvert0 with flex_vert0

`flex_xvert0` stored the Cartesian position of the vertices in `qpos0`. This array was unused, so it is now replaced with `flex_vert0`, which still contains the positions of the vertices, but normalized on the bounding box of the flex so that the coordinates are in `[0,1]^m->flex_dim`. This will be useful in the future for using different interpolation methods for computing flex deformations.

PiperOrigin-RevId: 691779361
Change-Id: I5c4223103cd4558f4e268fe3e8d8177541e4754f

introspect/structs.py

@@ -2603,11 +2603,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  array_extent=('nflexvert', 3),
              ),
              StructFieldDecl(
-                 name='flex_xvert0',
+                 name='flex_vert0',
                  type=PointerType(
                      inner_type=ValueType(name='mjtNum'),
                  ),
-                 doc='Cartesian vertex positions in qpos0',
+                 doc='vertex positions in qpos0 on [0, 1]^d',
                  array_extent=('nflexvert', 3),
              ),
              StructFieldDecl(