Relocate surface.get_slab_regions/center_slab as static methods for Slab, mark SlabGenerator.get_unreconstructed_slabs as private method

[DanielYang59]

Summary

• Relocate surface.get_slab_regions/center_slab as static methods for Slab, part of Fix surface.center_slab handling of Slab regions separated by vacuum  #4547, rationale: these methods are quite specific to Slab, make them methods for Slab for better grouping
• Mark SlabGenerator.get_unreconstructed_slabs as private method Relocate surface.get_slab_regions/center_slab as static methods for Slab, mark SlabGenerator.get_unreconstructed_slabs as private method #4548

Future TODO

• It's better if they're just instance methods, however they wouldn't work now if slab is Structure

[DanielYang59]

guess it's better to refactor to instance method at some point and avoid such intermeditate relocations