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

[fbeutel]

No description provided.

[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)