Nexus: Fix spglib compatibility

[kayahans]

Starting with v2.5.0 SPGLIB returns using SpglibDataset type from get_symmetry_dataset function, rather than using native dictionary. Current PR does type checking to transfer that information to Nexus and preserves compatibility.

What type(s) of changes does this code introduce?

• Bugfix

Does this introduce a breaking change?

• No: All tests in nxs-test pass. I downgraded spglib to 2.4.0 and upgraded to 2.6.0 then ran the tests after each case.

What systems has this change been tested on?

Python 3.12.8

Checklist

• Yes. This PR is up to date with current the current state of 'develop'
• No. Code added or changed in the PR has been clang-formatted
• No. This PR adds tests to cover any new code, or to catch a bug that is being fixed
• No. Documentation has been added (if appropriate)

[prckent]

FYI, with this fix I'll also update the spack package for py-spglib and try to get v2.6.0 installed and included in the nightly test configuration. EDIT: The spack package update looks to be non-trivial.

[ye-luo]

Test this please