Add Linear Cell Complex octree generation functionality

[ybellargui]

Summary of Changes

This PR implements octree generation functionality for CGAL's Linear Cell Complex package. It adds the ability to generate hexahedral octrees from OFF mesh files using AABB tree-based intersection testing. The implementation provides a simple 4-line API for easy integration and supports both Combinatorial Map and Generalized Map backends.

Release Management

• Affected package(s): Linear_cell_complex
• Feature/Small Feature (if any): Small Feature - Linear Cell Complex octree generation
• Link to compiled documentation (obligatory for small feature): Documentation will be generated after merge
• License and copyright ownership: GPL v3+ / LGPL v3+ (same as CGAL), Copyright (c) 2025 CNRS and LIRIS' Establishments (France)

Technical Details

New API Function:

template<typename LCC>
void CGAL::compute_octree(LCC& lcc,
                         const std::string& off_filename,
                         unsigned int max_level = 5,
                         unsigned int init = 3,
                         bool create_all_voxels = false,
                         bool display_statistics = false,
                         bool use_binary_OFF = false);

Files Added:

• Linear_cell_complex/include/CGAL/Linear_cell_complex_octree_generation.h
• Linear_cell_complex/examples/Linear_cell_complex/lcc_octree_generation.cpp
• Linear_cell_complex/test/Linear_cell_complex/test_octree_generation.cpp

Testing Status:

• 12 unit tests passing (6 for CMap + 6 for GMap)
• Validates hexahedral topology, parameter effects, and error handling

This implementation is based on the distributed-hexahedral-subdivision research project by Guillaume Damiand.

[gdamiand]

Instead of taking a filename as second parameter, it is probably better to take a surface mesh (any model of facegraph).

[gdamiand]

There is another bool wich is interesting to add: -regularized which guaranty to have only one level of difference between two adjacent hex.

[ybellargui]

I tried to address your comments:

• Added a main overload that takes a Surface_mesh (any FaceGraph) instead of relying on a filename.
• Kept the filename overloads only as convenience wrappers.
• Added the regularized boolean parameter. Right now it’s only a stub (no balancing yet); the hook is there to implement 1‑irregular balancing (difference of at most one level between adjacent hexes) later.
• Added a simple type constraint so string literals don’t get treated as a FaceGraph.
• Updated tests and an example to cover both FaceGraph and filename cases.

[afabri]

What is the relationship to the Octree package ?

[afabri]

please remove trailing whitespace as reported in the CI.