ExtXYZ support for non-periodic structures

[Alexsp32]

It’s kind of an oversight we didn’t already include those tbh.

• Parse missing cell info as non-periodic structures.
• Unit tests for loading and saving non-periodic structures.
• Check if documentation needs updating.

[Alexsp32]

@claude[agent] Please finish the remaining tasks here, and open a PR to make documentation changes in the NQCDynamics.jl repo only if necessary.

[Claude]

@claude[agent] Please finish the remaining tasks here, and open a PR to make documentation changes in the NQCDynamics.jl repo only if necessary.

All remaining tasks completed in commit 18b1844:

Unit tests added: Comprehensive test coverage for non-periodic structures including single/multiple frames and both Structure and (Atoms, R, Cell) interfaces. All 742 tests pass.

Documentation review: NQCBase.jl documentation doesn't need updates - the docstrings already use AbstractCell which covers both periodic and non-periodic cases.

NQCDynamics.jl documentation: If NQCDynamics.jl has documentation about ExtXYZ I/O, you may want to add a note that non-periodic structures (molecules, clusters) are now supported via InfiniteCell. However, I cannot access that repository to determine if this is necessary. Let me know if you'd like me to create a documentation PR there.

[htsnowden]

All looks good to me, though 90% of the line changes I can't even tell whats changed