Support optional smooth realspace cutoffs

[Copilot]

Realspace cutoffs previously introduced hard step functions in dispersion energies, which can create discontinuities in energies, gradients, and stress tensors as interatomic distances cross cutoff boundaries. This adds optional smoothing widths for two- and three-body dispersion cutoffs while preserving existing hard-cutoff behavior by default.

• Core dispersion

  • Added width2 and width3 to realspace_cutoff.
  • Applied smooth polynomial switching to two-body and ATM three-body dispersion contributions.
  • Included smooth cutoff derivative terms in gradients and virials.
  • Applied the same smoothing to pairwise dispersion representations across supported damping models.

• Public APIs

  • Added C API entry point:

    dftd3_set_model_realspace_cutoff_smooth(error, model, disp2, disp3, cn, width2, width3);

  • Extended Python DispersionModel.set_realspace_cutoff with optional width2 and width3.
  • Exposed ASE calculator support through realspace_cutoff.

• Documentation and coverage

  • Documented smooth cutoff widths in C, Fortran, and ASE-facing docs.
  • Added focused C, Fortran, and Python tests covering API exposure, gradient consistency, and pairwise energy consistency.

Example:

call get_dispersion( &
   mol, disp, param, &
   realspace_cutoff(disp2=60.0_wp, disp3=40.0_wp, cn=40.0_wp, &
                    width2=5.0_wp, width3=3.0_wp), &
   energy, gradient, sigma)

model.set_realspace_cutoff(
    disp2=60.0,
    disp3=40.0,
    cn=40.0,
    width2=5.0,
    width3=3.0,
)

[codecov]

Codecov Report

❌ Patch coverage is 34.44444% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.75%. Comparing base (9cbf93b) to head (962512c).

Files with missing lines	Patch %	Lines
src/dftd3/damping.f90	0.00%	16 Missing ⚠️
src/dftd3/damping/cso.f90	12.50%	1 Missing and 6 partials ⚠️
src/dftd3/damping/mzero.f90	14.28%	0 Missing and 6 partials ⚠️
src/dftd3/damping/optimizedpower.f90	14.28%	0 Missing and 6 partials ⚠️
src/dftd3/damping/rational.f90	14.28%	0 Missing and 6 partials ⚠️
src/dftd3/damping/zero.f90	14.28%	0 Missing and 6 partials ⚠️
src/dftd3/api.f90	37.50%	2 Missing and 3 partials ⚠️
src/dftd3/damping/atm.f90	20.00%	0 Missing and 4 partials ⚠️
src/dftd3/cutoff.f90	80.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #183      +/-   ##
==========================================
- Coverage   65.99%   65.75%   -0.25%     
==========================================
  Files          34       35       +1     
  Lines        4844     4885      +41     
  Branches     1672     1689      +17     
==========================================
+ Hits         3197     3212      +15     
- Misses        691      713      +22     
- Partials      956      960       +4     

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