sub-cell smoothing 1st implementation WIP

[Clara2223]

📝 Pull Request Summary

SCS implementation

• We voxelize in a grid that has $factor \times$ resolution, --> subdivides each cell in subcells
• We compute the gradient to have an idea of boundaries and make the mask continuous (not sure if this step is needed)
• We add an extra step of gaussian blurring to make the material/mask changes smooth without loosing accuracy

and then we generate an accumulated mask by combining the $factor$ masks (e.g.) if $factor=4$ we sum 4 sub masks.

Finally, we clip the values to the maximum of int8 = 255. Each value that has 255 is fully inside the solid. Every cell that has value<255 is a partially filled cell with a volume fraction value/255.

🔧 Changes Made

✅ Checklist

• Code follows the project's style and guidelines
• Added tests for new functionality
• Updated documentation (mentioned in docs/ or included in examples/ and notebooks/)
• Verified that changes do not break existing functionality (automatic tests passing)

📌 Related Issues / PRs

Closes #issue-number

[Copilot]

Pull request overview

This PR introduces an initial (WIP) implementation of sub-cell/subpixel smoothing for STL voxel masks and refactors SIBC (surface impedance boundary condition) handling for conductive solids in the FIT3D solver.

Changes:

• Added optional subpixel smoothing to STL voxelization in GridFIT3D and adjusted solver material-mask handling to accept non-binary (float) masks.
• Refactored SIBC application into a dedicated _apply_SIBC() method and changed the default use_sibc behavior in SolverFIT3D.
• Updated an MPI lossy cavity test fixture to include an explicit sigma=0.0 entry for vacuum materials.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
wakis/solverFIT3D.py	Changes default SIBC behavior, updates STL material masking to float masks, and adds/refactors SIBC application logic.
wakis/gridFIT3D.py	Adds subpixel smoothing options and implementation for voxelized STL masks; adjusts surface-mask handling for SIBC.
tests/test_007_mpi_lossy_cavity.py	Updates STL material input format to include explicit conductivity for vacuum.

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[Clara2223]

:D