CI: update CIFs monthly

.github/workflows/cifs.yml

@@ -0,0 +1,37 @@
+name: Update CIFs
+
+on: 
+  push:
+  schedule:
+    # Every month
+    - cron: '0 0 1 * *'
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Set-up git
+      run: |
+        git config user.name "GitHub Actions"
+        git config user.email "<>"
+
+    - name: Update CIFs
+      run: |
+        python -m pip install --upgrade pip
+        pip install numpy wheel
+        pip install -r requirements.txt
+      
+        python -m get_cifs
+
+    - name: Commit changes if any
+      run: |
+        git add crystals/cifs/*.cif
+        if [ -n "$(git status --porcelain)" ];
+        then
+          git commit --message "Updated CIFs"
+          git push origin master
+        fi