Make the import_cmip6_data notebook functional and documented thanks to the improvements made elsewhere.

Issue

Solving issue #3 (closed), I've noticed that in order to comply to the current milestone many changes could be done inside the notebook.

Goals

• Solve issue #11 (closed).
• Solve issue #20 (closed).
• Solve issue #21 (closed).
• Solve issue #22 (closed).
• Solve issue #36 (closed).
• Solve issue #40 (closed).