Add smooth analytic ZBL correction

[Pragalbhv]

Summary

Adds ZBLCorrection, a fixed (non-trainable), differentiable short-range Ziegler–Biersack–Littmark repulsive correction as a CACE output_module.
Prevents unphysical atomic collapse at very short range without perturbing normal-bond-length behavior.

Design

• Reproduces the ZBL formula and numerical constants from hiphive's ZBLCalculator (https://gitlab.com/materials-modeling/hiphive), ported to PyTorch on CACE's directed-edge graph (edge_index/shifts) so it is differentiable through CACE's existing Forces/stress pipeline via autograd — no separate calculator-side implementation needed.
• Smoothly switched to exactly zero above a configurable switch_off using the existing cace.modules.cutoff.SwitchFunction, so it only affects the short-range repulsive wall and leaves the learned + electrostatic energy untouched at normal interatomic distances.
• Correctly accounts for CACE's directed (doubled) edge list (0.5 per-edge weighting), batched multi-structure graphs (scatter_sum over data["batch"]), and periodic images (including self-loop edges through nonzero periodic shifts).
• No trainable parameters; drops into an existing output_modules list ahead of FeatureAdd/Forces with no changes to either.

Usage

zbl = cace.modules.ZBLCorrection(switch_on=..., switch_off=..., output_key="zbl_energy")
e_add = cace.modules.FeatureAdd(
    feature_keys=[..., "zbl_energy"], output_key="CACE_energy"
)
output_modules = [..., zbl, e_add, forces]

Tests

tests/test_zbl.py (9 tests, all passing):

• Analytic pair energy vs. hiphive's closed-form value, with directed-edge (i→j/j→i) double-counting accounted for.
• Switch-to-zero above switch_off.
• Force gradient vs. finite-difference/autograd of the energy.
• Finite energy/gradient at near-zero distance in float32 and float64.
• Correct ordering through FeatureAdd → Forces.
• Constructor validation (switch_off vs. cutoff).
• Periodic self-loop pairs (atom interacting with its own periodic image) treated as ordinary weighted pairs.
• Batching: per-structure energies match independent evaluation.