Black formatter

[ewels]

Reformatted all Python code using Black and set up tests and documentation.

I also reformatted the main Readme quite a bit to adhere to standard Markdown syntax (eg. language specification for code colouring in code blocks, surrounding code blocks with newlines etc - it's possible to also lint markdown style if you want to go down this route, see markdownlint-cli and an example GitHub Actions CI workflow). You can see how this is rendered on GitHub on my fork.

Closes https://github.com/bionitio-team/bionitio-python/issues/6