Add CITATION.cff file

Closes #70 (closed).

• Add CITATION.cff file to the root of the project.

Important note

At the moment, the conversion from CITATION.cff to metacode.json and planetary-coverage.bib can not be performed with cffconvert. To many fields are missing in cff file format. We recommend to keep all the 3 files for now.

Maintenance

• Update bump2version configuration to include CITATION.cff.
• Bump swh-id for the latest release.
• Bump spiceypy dependency to 6.0.0
• Bump Pillow dependency to 10.0.0
• Bump dependencies to the latest version (except flake8-isort due to pkg_resources error, reported into gforcada/flake8-isort#141)
• Upgrade poetry dependency to 1.6.
• Fix Numpy depreciation warning