Function cross_sections_on_grid 

pub fn cross_sections_on_grid(
    data: &ResonanceData,
    energies: &[f64],
) -> Vec<CrossSections>

Compute cross-sections over a grid of energies.

Optimized batch evaluation: precomputes J-groups and per-resonance invariants (reduced width amplitudes, penetrability at E_r) once per resonance range, then evaluates each energy point using the cached data. This avoids redundant group_by_j + penetrability(l, rho_r) calls that the per-point API (cross_sections_at_energy) would repeat.

Issue #87: the precompute is hoisted above the energy loop so that precompute_jgroups_* runs O(ranges) times total, not O(ranges × energies).

Limitations

MLBW (Multi-Level Breit-Wigner, LRF=2) ranges use true MLBW with interference. formulas as an approximation, ignoring resonance-resonance interference. Results may be inaccurate for closely spaced or overlapping resonances.

Arguments

• data — Parsed resonance parameters from ENDF.
• energies — Slice of neutron energies in eV.

Returns

Vector of cross-sections, one per energy point.