Poetry migration

Close #50 (closed)

• Add pyproject.toml
• Move the source files to a src/ folder
• Remove setup.py, setup.cfg and MANIFEST.in
• Release pinned-version dependencies
• Upgrade dependencies versions
• Update README.md local dev install recommendations to use Poetry.
• Update gitlab-ci.yml, .readthedocs.yaml and docs/makefile for Poetry config.
• Update docs examples and pin JUICE version to v300 (to avoid #51 (closed) issue).
• Improve installation document section (#50 (closed))
• SpicePool coverage windows are evaluated per-file and not per-window.
• Add a script to trim ck and spk kernels files used in the docs.
• Bump JUICE SKD version (v320_20220725_001) in the tests.
• Fix colorbar ticks with the latest version of matplotlib (3.5.2) reported in !18 (merged) .
• Migrate to spiceypy 5.1.1 (to fix Apple Silicon issue).