Labels missing when using export_step on nested compounds.

[jsmnbom]

When nesting compounds are used, the labels are lost when exporting.

Example:

cube = Box(1,1,1)
cube.label = 'Cube'

arm = Compound(
  label="Arm",
  children=[
    copy.copy(cube),
  ]
)

assembly = Compound(
  label="Root",
  children=[
    copy.copy(arm),
    copy.copy(arm).locate(Pos(10,0,0))
  ]
)

print(assembly.show_topology())
build123d.export_step(assembly, 'build/assembly.step')

Shows correct topology tree:

Root         Compound at 0x74cc4dde22b0, Location(p=(0.00, 0.00, 0.00), o=(-0.00, 0.00, -0.00))
├── Arm      Compound at 0x74cc4dde1db0, Location(p=(0.00, 0.00, 0.00), o=(-0.00, 0.00, -0.00))
│   └── Cube Box      at 0x74cc4de24a10, Location(p=(0.00, 0.00, 0.00), o=(-0.00, 0.00, -0.00))
└── Arm      Compound at 0x74cc4dde2850, Location(p=(10.00, 0.00, 0.00), o=(-0.00, 0.00, -0.00))
    └── Cube Box      at 0x74cc4de24ad0, Location(p=(0.00, 0.00, 0.00), o=(-0.00, 0.00, -0.00))

But if opened in any external program the step file looks more like this:

Applying the fix in #953 gets us closer to the correct result:

Unfortunately it still leaves the Solids unnamed which is not optimal.
Ideally it would end up like this:

[jsmnbom]

I'm using the below code and so far it seems to function well. Leaving here since I don't wanna make too many draft PR's and it would fit well in a PR refactoring export/import code more broadly see my comment in #942 (comment).

def _add_shape(
  shape_tool: XCAFDoc_ShapeTool,
  color_tool: XCAFDoc_ColorTool,
  shape: Shape,
) -> TDF_Label:
  is_assembly = isinstance(shape, Compound) and len(shape.children) > 0
  label = self.shape_tool.AddShape(
      shape.wrapped,
      is_assembly,
      True,
  )

  for node in PreOrderIter(shape):
    found = shape_tool.FindShape(node.wrapped)
    if not found.IsNull():
      instances = TDF_LabelSequence()
      shape_tool.GetUsers_s(found, instances)
      children = get_non_assembly_children(shape_tool, found)
      for label in list(children) + list(instances) + [found]:
        _add_tdf_data(color_tool, label, node)

def _add_tdf_data(
  color_tool: XCAFDoc_ColorTool,
  label: TDF_Label,
  node: Shape | None = None,
):
  if node.label:
    TDataStd_Name.Set_s(label, TCollection_ExtendedString(node.label))

  if node.color:
    color_tool.SetColor(label, node.color.wrapped, XCAFDoc_ColorType.XCAFDoc_ColorSurf)

def get_non_assembly_children(
  shape_tool: XCAFDoc_ShapeTool,
  label: TDF_Label,
  children: TDF_LabelSequence | None = None,
):
  if children is None:
    children = TDF_LabelSequence()
  it = TDF_ChildIterator(label)
  while it.More():
    child_label = it.Value()
    ref_label = TDF_Label()
    if shape_tool.GetReferredShape_s(child_label, ref_label):
      if not shape_tool.IsAssembly_s(ref_label):
        children.Append(child_label)
        children.Append(ref_label)
        get_non_assembly_children(shape_tool, ref_label, children)
    else:
      if not shape_tool.IsAssembly_s(child_label):
        children.Append(child_label)
    it.Next()
  return children