Adding nbformat.

Author: vitenti

.github/workflows/publish.yml

@@ -36,7 +36,7 @@ jobs:
           use-mamba: true
       - name: Create environment
         run: |
-          mamba create -v -q -n numcosmo numcosmo jupyter
+          mamba create -v -q -n numcosmo numcosmo jupyter nbformat
       - name: Activate environment
         run: conda activate numcosmo
       - name: Render and Publish