Add mio as a selectable DFTB parameter set

[galjos]

Adds a parameter_set selector so a run can choose between the 3ob (DFTB3) and mio (DFTB2) Slater-Koster sets, while still needing only a structure + charge (+ optional spin) — spin polarisation and constants follow from the set automatically.

What changed

• calculator/dftbplus.py: new dftb_mio_parameters (second-order model — no ThirdOrderFull/Hubbard derivatives) and a DFTB_PARAMETER_SETS registry resolved via resolve_parameter_set(). _spin_kwargs now accepts the spin-constant table so each set supplies its own.
• thermo/api.py, thermo/screening.py, cli/thermo.py: thread the selection through dftbplus_thermo, screen, and the CLI (thermo screen --parameter-set 3ob|mio).
• cli/dftb_setup.py: generalise the Slater-Koster downloader — thermo setup-dftb --parameter-set mio fetches mio-1-1 from its release.

mio spin constants

mio is the LDA-based Elstner-1998 set and ships no spin constants of its own. The atomic spin constant is only weakly functional-dependent for H/C/N/O/S, and the only well-documented organic values (3ob's spinw.hsd, matching the DFTB+ manual's own H₂O example: H -0.072, O Wpp -0.028) are the authoritative choice — so mio reuses the 3ob constants. The real difference between the sets is the Hamiltonian (DFTB2 vs DFTB3).

Validation

• Real DFTB+ end-to-end via screen(parameter_set="mio"): an OH radical runs colinear spin-polarised (DFTB2, UnpairedElectrons = 1, constants H -0.07174 / O -0.02785) and yields sensible thermochemistry; 3ob is unchanged.
• Automatic download confirmed against the live mio-1-1 release (36 .skf files).
• Offline suite green (188 passed); real-DFTB+ integration tests green with DFTB_PREFIX set (29 passed, 0 skips).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.31%. Comparing base (d51d08a) to head (dc75c0b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
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+ Coverage   96.23%   96.31%   +0.08%     
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  Files          22       22              
  Lines        1328     1358      +30     
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+ Hits         1278     1308      +30     
  Misses         50       50              

Flag	Coverage Δ
unittests	96.31% <100.00%> (+0.08%)	⬆️

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