Stop mapping materials for voxel geometry views

- Voxel geometry views do not use compound/averaged materials. Therefore
  materials are guaranteed to be correctly ordered in both the serial
  and MPI implementations as the materials used have all be added from
  the Scene/input file.

gprMax/output_controllers/geometry_view_voxels.py

@@ -52,11 +52,11 @@ class GeometryViewVoxels(GeometryView[GridType]):
 
         self.spacing = self.grid_view.step * self.grid.dl
 
+        self.nbytes = self.material_data.nbytes
+
         # Write information about any PMLs, sources, receivers
         self.metadata = Metadata(self.grid_view)
 
-        self.nbytes = self.material_data.nbytes
-
     def write_vtk(self):
         """Writes geometry information to a VTKHDF file."""
 
@@ -82,10 +82,6 @@ class MPIGeometryViewVoxels(GeometryViewVoxels[MPIGrid]):
         self.origin = self.grid_view.global_start * self.grid.dl
         self.spacing = self.grid_view.step * self.grid.dl
 
-        # Use global material IDs rather than local IDs
-        self.grid_view.initialise_materials(filter_materials=False)
-        self.material_data = self.grid_view.map_to_view_materials(self.material_data)
-
         self.nbytes = self.material_data.nbytes
 
         # Write information about any PMLs, sources, receivers

gprMax/output_controllers/geometry_views.py

@@ -243,13 +243,18 @@ class Metadata(Generic[GridType]):
         return self.grid.size
 
     def materials_comment(self) -> Optional[List[str]]:
-        if self.grid_view.materials is None:
+        if hasattr(self.grid_view, "materials"):
+            materials = self.grid_view.materials
+        else:
+            materials = self.grid.materials
+
+        if materials is None:
             return None
 
         if not self.averaged_materials:
-            return [m.ID for m in self.grid_view.materials if m.type != "dielectric-smoothed"]
+            return [m.ID for m in materials if m.type != "dielectric-smoothed"]
         else:
-            return [m.ID for m in self.grid_view.materials]
+            return [m.ID for m in materials]
 
 
 class MPIMetadata(Metadata[MPIGrid]):