Chip.add_cell: allow specifying the magnification of the added cell

[fbeutel]

[HelgeGehring]

Thanks! I think I left this parameter out, as not all functions are considering the value yet. It should be implemented in

• [ ] get_bonds
• [ ] get_reduced_layer
• [ ] save
• [ ] get_patches to not cause inconsistencies. Before we add the parameter, I think we should implement it (or add a warning to the functions which ignore magnification if magnification is used)