Add conformer-ensemble thermochemistry helpers

[galjos]

What

Adds ThermoScreening.thermo.ensemble, three pure post-processing helpers that combine the absolute Gibbs free energies (Thermo.total_EeGtot(), in Hartree) of several conformers of the same molecule into ensemble properties. Compute each conformer with matching engine/conditions (generate → xtb_cli_thermo/…), then combine:

• boltzmann_weights(thermos, temperature=298.15) — normalised conformer populations, evaluated with a min-energy shift for numerical stability.
• ensemble_free_energy(thermos, temperature=298.15, unit="H") — the Boltzmann-averaged free energy -k_B T ln( Σ exp(-G_i / k_B T) ), which sits at or below the lowest conformer's free energy by the conformational mixing entropy. Units H/eV/kcal/kJ.
• lowest_gibbs(thermos) — the single lowest-free-energy conformer (to carry a dominant structure forward, e.g. into reaction_free_energy).

All three are exported from ThermoScreening.thermo. This is the natural consumer of the existing conformer generation + screening features.

Refactor

Factors the shared Hartree → eV/kcal/kJ conversions out of reactions.py into a small private _units module (convert_from_hartree), so both post-processing modules use one source of truth. reactions.py public behaviour is unchanged.

Tests

Exact-math and physics checks: degenerate populations (0.5/0.5), single-conformer identity, normalisation + ordering, a known k_B*T-gap ratio (e:1), single-conformer ensemble = its own G, degenerate mixing entropy (G_ens = G - k_B T ln 2 < G), ensemble ≤ minimum, unit conversion, and edge cases (empty ensemble, non-positive temperature, unknown unit).

Docs

API-reference entries plus a "Conformer ensembles" usage section with a worked n-butanol example.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.92%. Comparing base (89dd177) to head (ba3c8ef).

Additional details and impacted files

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+ Coverage   96.85%   96.92%   +0.06%     
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  Files          26       28       +2     
  Lines        1655     1689      +34     
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+ Hits         1603     1637      +34     
  Misses         52       52              

Flag	Coverage Δ
unittests	96.92% <100.00%> (+0.06%)	⬆️

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