add dagmc fill materials to homogenized materials method (#3026)

Co-authored-by: Paul Romano <[email protected]>

openmc/mesh.py

@@ -211,6 +211,18 @@ def get_homogenized_materials(
 
         # Create homogenized material for each element
         materials = model.geometry.get_all_materials()
+
+        # Account for materials in DAGMC universes
+        # TODO: This should really get incorporated in lower-level calls to
+        # get_all_materials, but right now it requires information from the
+        # Model object
+        for cell in model.geometry.get_all_cells().values():
+            if isinstance(cell.fill, openmc.DAGMCUniverse):
+                names = cell.fill.material_names
+                materials.update({
+                    mat.id: mat for mat in model.materials if mat.name in names
+                })
+
         homogenized_materials = []
         for mat_volume_list in mat_volume_by_element:
             material_ids, volumes = [list(x) for x in zip(*mat_volume_list)]