Simple integration of MolBar into OPI´s Structure Object

[haneug]

Is your feature request related to a problem? Please describe.
I would like to interface OPI Structure objects with MolBar (https://git.rwth-aachen.de/bannwarthlab/molbar), a powerful molecular identifier.

Describe the solution you'd like
A method structure.to_molbar() that delegates to a utility function in opi/utils/molbar.py.
The utility module handles the lazy import of MolBar and hands the coordinates to Molbar with the to_xyz_block(). Secondly, we could add a standalone molbar_data_to_structure() utility function that reconstructs a Structure from a MolBar.

MolBar will not be added as a hard dependency. The utility module should raise a clear ImportError with an install hint if MolBar is not present, following the same pattern used for ASE.

Describe alternatives you've considered
We could add MolBar as a dependecy but I would not like to do that.

[crjacinto]

On it

[timmyte]

@haneug
As Molbar has MIT license we can use without restrictions.

I would add this as optional dependency (uv add --optional molbar -- molbar).
Thereby it's not installed by default, but can be installed using:

uv sync --extra molbar
# or
pip install 'orca-pi[molbar]'