Verify import_cmip6_data.ipynb compliance with the milestones' criteria

Issue

I want to check and improve the notebook's clarity.

Goals

• The notebook has an objective section
• The notebook has a conclusion section
• The notebook is organized in logical processing steps, with an explicit narrative structure, with each cells having a clear purpose, and including comments/intermediate conclusions between cells
• Each Jupyter cell has a short comment that describes the purpose of the cell (doc strings, headers)