Support for differeent wrap modes in the systems API

We should provide support for handling different wrapping modes directly in the API.

Tasks:

• Modify API request model to include new wrap_mode parameter with fixed options: wrap, unwrap, or 'raw'.
• Add functions for wrap and unwrap directly in atomutils. These should work on positions, cell and pbc.
• Create tests
• Add GUI option in the system download menu. By default the mode should be 'raw', but can be selected from a dropdown. Needs good descriptions.