get_xspectra_structures: Refactor and Improve Code (#1026)

This PR refactors the `get_xspectra_structures` calcfunction to enable users to set symmetry data manually
instead of relying on automatic symmetry analysis.
* Moves supercell creation, processes for molecules, and generation of  `equivalent_sites_data` to separate functions.
* Adds functionality for users to manually provide symmetry data, thus enabling the `CalcFunction` to be used as a means to generate structures with user control over which exact sites to mark.
* Fixes a bug discovered in the case where non-hubbard structures with custom Kind names lost their Kind names when converting the ASE supercell to `StructureData` type.
* Fixes a small oversight in the tests where `spglib_settings` was mistakenly named `spglib_options`.