Update set-up

See merge request !1

Modernize everything, in particular, use Python virtual environments, use BuildKit to build Docker images, upgrade dependencies, install package in build step, use `.env` file to customize environment, and improve test markers

Add step explaining how to add SSH key to GitLab and how to add it to the authentication agent `ssh-agent` (needed to clone repository via SSH which is done in the subsequent step)

Use pip and python version 3 explicitely, prefix paths with ./, and use `pytest` instead of `python3 -m pytest`

Use the soon to be default import mode `importlib` (see https://docs.pytest.org/en/6.2.x/pythonpath.html#import-modes)

Install package in editable mode with pip (so that it can be imported properly without changing `sys.path`)

Say how to deactivate a virtual environment using the shell alias `deactivate` that is defined by the activate script

Use virtual environments in the installation process without Docker and tell how `requirements-development.txt` can be used to install the development tools easily

Put Python development tools that can be installed with pip in the file `requirements-development.txt`

Add detailed explanations on how to set-up a working project envrionment with and without Docker, improve and expand the reading list, and explain the process of how to make code changes in detail

Add the module calculator with the two methods `raize` and `_hidden`, corresponding documentation, doctests and pytests