Prepare release 0.9.0

[davide-f]

Closes # (if applicable).

Changes proposed in this Pull Request

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• Unit tests for new features were added (if applicable).
• A note for the release notes docs/release_notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license and have added my name to the docs/contributors.md.

[davide-f]

@fneum I've drafted the package for the new release.
Would you envision to include #311 in the new release?

I'll be waiting comments before proceeding.
After I'd have green light, I'd merge this PR with necessary modification/updates and proceed with the release (tagging etc), unless of different advise