Config-driven standard path/filename for SU(N) symmetry data

[LudwigZw]

Summary

• Add su_n_data_path / su_n_data_filename_base cyten config options, defaulting to the
  literal /home/<login>/.tenpy/su_n_symmetry_data convention (matching the external
  clebsch_gordan_coefficients generator) on all platforms, overridable via set_options,
  CYTEN_SU_N_DATA_PATH/CYTEN_SU_N_DATA_FILENAME_BASE, or .cytenconfig.yaml.
• Add su_n_data_filename() / su_n_data_file_path() helpers implementing the standard
  <base>_N{N}_{CG|F|R}_hweight{H}.hdf5 naming convention, exposed via cyten.symmetries.
• Add SUN(N, hweight, *, cg_hweight=None, f_hweight=None, r_hweight=None, path=None, filename_base=None, descriptive_name=None) -- resolves and opens the three standard files
  directly, validating CG hweight >= F/R hweight and raising a FileNotFoundError that names
  every override option when a file is missing. The existing SUN(N, CGfile, Ffile, Rfile, ...)
  handle-based constructor is unchanged.
• Fix home_directory() to fall back to USERPROFILE/HOMEDRIVE+HOMEPATH on Windows (it
  previously only read $HOME, so the documented ~/.cytenconfig.yaml silently never loaded there).

Test plan

• pytest tests/python_tests/test_config.py -q -- 6 passed
• pytest tests/python_tests/test_symmetries.py -q -- 51 passed, 2 skipped (no local SU(N)
  .hdf5 data files, as before)
• ruff check / flake8 clean on touched files
• Manual smoke test: su_n_data_file_path(3, 'CG', 7) resolves to
  /home/<user>/.tenpy/su_n_symmetry_data/su_n_clebsch_gordan_data_N3_CG_hweight7.hdf5

[jhauschild]

I cleaned this up and merged it with the latest main branch.
@LudwigZw I checked that tests run locally, but (just like the github action) only without the generated hdf5 files in the correct locations. Can you please check that all tests pass if you actually do have the data? If that is the case, you can merge.