Improves Pseudolikelihood computation performance

[Freguglia]

No description provided.

[Copilot]

Pull request overview

This PR improves the performance of pseudo-likelihood computation in the mrf2d package by (1) refactoring the per-pixel conditional probability computation to use a log-space, log-sum-exp formulation that avoids per-pixel NumericVector allocation and gives better numerical stability, (2) reordering the inner loops to iterate over neighbors outermost and values innermost (better cache locality plus a single bounds check per direction), and (3) adding optional OpenMP parallelization over the pixel grid in log_pl_mrf / log_pl_mrf_sub.

Changes:

• Rewrite conditional_probabilities_mrf{,_sub} to accumulate log-potentials and normalize with the log-sum-exp trick.
• Add log_cond_pixel{,_sub} helpers and OpenMP parallel for collapse(2) reductions in log_pl_mrf{,_sub}, with thread-local scratch buffers.
• Add src/Makevars and src/Makevars.win enabling SHLIB_OPENMP_CXXFLAGS; bump Version to 1.0.2, RoxygenNote to 7.3.3, and actions/upload-artifact to v4.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/pseudo_likelihood.cpp	New log-space per-pixel helpers and OpenMP-parallel reductions over the grid.
src/conditional_probability.cpp	Reordered loops and added log-sum-exp normalization for numerical stability.
src/Makevars	New file enabling OpenMP flags on Unix builds.
src/Makevars.win	New file enabling OpenMP flags on Windows builds.
DESCRIPTION	Bumps package Version and RoxygenNote.
.github/workflows/test-coverage.yaml	Upgrades upload-artifact action from v3 to v4.

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