Fix visualization test using deprecated PyVista method (#433)

Replace the deprecated `pv_grid.cell_bounds()` for an iterator over the
cells (`pv_grid.cell`) and then use `Cell.bounds` to get the bounds of
each cell.

harmonica/tests/test_visualizations.py

@@ -77,8 +77,8 @@ def test_prism_to_pyvista(prisms, density):
  assert pv_grid.n_cells == 4
  assert pv_grid.n_points == 32
  # Check coordinates of prisms
- for i, prism in enumerate(prisms):
- npt.assert_allclose(prism, pv_grid.cell_bounds(i))
+ for prism, cell in zip(prisms, pv_grid.cell):
+ npt.assert_allclose(prism, cell.bounds)
  # Check properties of the prisms
  assert pv_grid.n_arrays == 1
  assert pv_grid.array_names == ["density"]