Add CI workflow for performance benchmarks

This commit introduces a new GitHub Actions workflow for performance benchmarking. The workflow is triggered on `pull_request_target` events and can be triggered with a "/rerun-benchmarks" comment in the PR. It should be compatible with PRs from both forks and the main repository. It includes steps for setting up the environment, checking out the PR and main branches, installing dependencies, and running benchmarks on both branches. The results are then compared, encoded, and posted as a comment on the PR.

.github/workflows/benchmarks.yml

@@ -0,0 +1,77 @@
+name: Performance benchmarks
+
+on:
+  pull_request_target:
+    types: [opened, ready_for_review]
+    branches:
+      - main
+  issue_comment:
+    types: [created]
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  run-benchmarks:
+    if: >
+      github.event_name == 'pull_request_target' ||
+      (github.event_name == 'issue_comment' && contains(github.event.comment.body, '/rerun-benchmarks'))
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout PR branch
+        uses: actions/checkout@v4
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+      - name: Add project directory to PYTHONPATH
+        run: echo "PYTHONPATH=$PYTHONPATH:$(pwd)" >> $GITHUB_ENV
+      - name: Install dependencies
+        run: pip install numpy pandas tqdm tabulate
+      - name: Checkout main branch
+        uses: actions/checkout@v4
+        with:
+          ref: main
+          repository: EwoutH/mesa
+      - name: Run benchmarks on main branch
+        working-directory: benchmarks
+        run: python global_benchmark.py
+      - name: Upload benchmark results
+        uses: actions/upload-artifact@v4
+        with:
+          name: timings-main
+          path: benchmarks/timings_1.pickle
+      - name: Checkout PR branch
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+          fetch-depth: 0
+          persist-credentials: false
+          clean: false
+      - name: Download benchmark results
+        uses: actions/download-artifact@v4
+        with:
+          name: timings-main
+          path: benchmarks
+      - name: Run benchmarks on PR branch
+        working-directory: benchmarks
+        run: python global_benchmark.py
+      - name: Run compare timings and encode output
+        working-directory: benchmarks
+        run: |
+          TIMING_COMPARISON=$(python compare_timings.py | base64 -w 0)  # Base64 encode the output
+          echo "TIMING_COMPARISON=$TIMING_COMPARISON" >> $GITHUB_ENV
+      - name: Comment PR
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const output = Buffer.from(process.env.TIMING_COMPARISON, 'base64').toString('utf-8');
+            const issue_number = context.issue.number;
+            github.rest.issues.createComment({
+              issue_number: issue_number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: 'Performance benchmarks:\n\n' + output
+            });