Add DRESS Monte Carlo integration for DT and DD thermal neutron spectra

[Copilot]

Integrates pydress as an optional backend for computing primary neutron spectra, complementing the existing Ballabio/Brysk analytical fits with a full Monte Carlo treatment. Supports separate deuteron and triton temperatures for DT — not possible with the scalar Ballabio moments.

Changes

• dress_interface.py — Replaces the preliminary stub with three functions:

  • DRESS_DT_spec(T_D, T_T, n_samples, bins) — DT spectrum with independent D/T temperatures
  • DRESS_DD_spec(T, n_samples, bins) — DD thermal spectrum
  • DRESS_DT_spec_single_T — thin backward-compatible wrapper
  • Fixes in-place mutation bug (bins /= 1e3) from the original stub; adds type hints; raises a clear ImportError when pydress is absent

• core.py — Two new public routines (eV throughout, matching NeSST conventions):

  • QDress_DT(Ein, T_D, T_T=None, n_samples=1e6) — T_T defaults to T_D
  • QDress_DD(Ein, Tion, n_samples=1e6)

• __init__.py — dress import wrapped in try/except; NeSST loads normally without pydress.

• pyproject.toml — pydress added as optional [dress] extra.

• tests/test_dress.py — Five new tests comparing DRESS moments (mean ±1 %, σ ±5 %) against Ballabio for DT and DD, plus a check that asymmetric T_D ≠ T_T produces a different spectrum.

Usage

import numpy as np
import NeSST as nst

Tion = 5e3  # eV
mean, stddev, _ = nst.DTprimspecmoments(Tion)
Ein = np.linspace(mean - 10*stddev, mean + 10*stddev, 500)

# Equal temperatures — equivalent to single-T Ballabio case
spec_eq = nst.QDress_DT(Ein, Tion)

# Separate D and T temperatures (not possible with Ballabio)
spec_asym = nst.QDress_DT(Ein, T_D=5e3, T_T=10e3)

# DD
spec_dd = nst.QDress_DD(Ein_dd, Tion)

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