Add a GHA to enforce version bump

.github/workflows/check_fits_version.yaml

@@ -0,0 +1,31 @@
+name: Check that the version number was bumped if _fits.py was modified
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  check_version:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v2
+
+    - name: Get previous version
+      id: get_previous_version
+      run: echo "::set-output name=version::$(git show HEAD^:python/lsst/cell_coadds/_fits.py | grep 'MODULE_VERSION =' | cut -d'=' -f2 | tr -d '[:space:]')"
+
+    - name: Get current version
+      id: get_current_version
+      run: echo "::set-output name=version::$(grep 'MODULE_VERSION = ' python/lsst/cell_coadds/_fits.py | cut -d'=' -f2 | tr -d '[:space:]')"
+
+    - name: Compare versions
+      id: compare_versions
+      run: |
+        if [[ "${{ steps.get_previous_version.outputs.version }}" == "${{ steps.get_current_version.outputs.version }}" ]]; then
+          echo "Error: _fits.py was modified but MODULE_VERSION was not!"
+          exit 1
+        fi