Add linting and testing as part of the CI to force consistent coding styles

From now on, it is only possible to merge correctly linted and formatted code to the master branch. Furthermore, unit test need to be passed.

Make sure to read the contributing guidelines before creating a merge request. The most important things are the follwing:

1. That you correctly install your development environment. If you already have python installed you just

pip install -e ".[dev, examples]"

Remarks: In the README.md you also find instructions for the case you don't have python and want to install an enviroment with conda.

2. Before you create a merge request, you can check if the linting and tests are passed by

invoke lint && invoke tests

You can use invoke format to reformat your files.