Write geometry view metadata before material data

- Writing metadata requires collective communication. This should be
  completed first to avoid ranks having to wait for all other ranks to
  complete the longer I/O operation of writing out the material data.

gprMax/output_controllers/geometry_view_lines.py

@@ -181,5 +181,5 @@ class MPIGeometryViewLines(GeometryViewLines[MPIGrid]):
             self.cell_offsets,
             comm=self.grid_view.comm,
         ) as f:
-            f.add_cell_data("Material", self.material_data, self.grid_view.offset)
             self.metadata.write_to_vtkhdf(f)
+            f.add_cell_data("Material", self.material_data, self.grid_view.offset)

gprMax/output_controllers/geometry_view_voxels.py

@@ -99,5 +99,5 @@ class MPIGeometryViewVoxels(GeometryViewVoxels[MPIGrid]):
             self.spacing,
             comm=self.grid_view.comm,
         ) as f:
-            f.add_cell_data("Material", self.material_data, self.grid_view.offset)
             self.metadata.write_to_vtkhdf(f)
+            f.add_cell_data("Material", self.material_data, self.grid_view.offset)