Add atom-specific scattering form factors

[mlund]

Summary

• Add scattering_f0 property to AtomData for atom-specific scattering form factors (defaults to 1.0)
• All scattering schemes now support form factors:
  • Debye: uses FormFactorAtomicConstant
  • EXPLICIT_PBC: uses FormFactorAtomicConstant
  • EXPLICIT_IPBC: uses FormFactorAtomicConstant
• Normalize scattering intensity by Σfᵢ² (sum of squared form factors) instead of N
• Backwards compatible: when all scattering_f0 = 1.0, normalization is identical to before

Usage

atomlist:
  - C: { sigma: 3.4, scattering_f0: 6.0 }
  - N: { sigma: 3.2, scattering_f0: 7.0 }

Test plan

• Unit tests for FormFactorAtomicConstant
• Unit tests for scattering_f0 JSON serialization
• Verify backwards compatibility (default f0=1 gives same results)
• All scattering schemes use form factors