Upgrade notebook syntax

• Simplify a bit the notebooks with the latest planetary_coverage syntax (%load_ext planetary_coverage).
• Use list comprehension instead of single list append.
• Fix missing imports.
• Simply cosmographia json export.
• Add an explicit .gitignore in temp/
• Use overwrite=True in agm ck save to file.

I think there is an issue in power-optmised-attitude.ipynb cell [15] (see below).