add missing script to ci

.gitlab-ci.yml

@@ -26,7 +26,8 @@ build and upload the contents:
     # Compile lectures
     - python code/band_structures.py
     - python code/bands_2d.py
+    - python code/heat_capacity.py
     - mkdocs build
     - "rsync -rv site/* solidstate@tnw-tn1.tudelft.net:"
   only:
-    - branches@solidstate/lectures
\ No newline at end of file
+    - branches@solidstate/lectures