Kallisto

Maintenance and bugfix release.
Thanks to RMeli and awvwgk

- Bugfix: remove [lgtm.com](https://lgtm.com/projects/g/AstraZeneca/kallisto) alerts (806c2e1, 7d02fdf) 🐞
- [JOSS](https://github.com/openjournals/joss-reviews/issues/3050): simplify installation instructions (d8420d8)
- Remove flake8 dependency from [tool.poetry.dependencies]

Maintenance and bugfix release.
Thanks to awvwgk

- Bugfix: remove hardcoded constraint in writeTransitionMetalConstrains (a1fed6b)🐞
- Bugfix: Fix broken link in CONTRIBUTING.md (9d754e5) 🐞

- Refactor: move large parts of kallisto to its own directory (4a2df26)
- Improved unit test coverage for atom (d2368c5)
- Improved unit test coverage for molecule (3aa9f97)
- Improved unit test coverage for xyz (9789090)
- Improved unit test coverage for turbomole (105ac9a)
- Improved unit test coverage for strucreader (ff5c2a3)

First GitHub release of the ``kallisto`` program :moon:, which is now open source :fireworks:
Check out the [online documentation](https://ehjc.gitbook.io/kallisto/) :book: for a complete description of all high-level functionalities.