Address code style issues in the existing code base

[CrsiX]

Issue #38 has added a CI pipeline to check for Java style guide violations. However, the existing problems (look here) are not addressed yet. This issue aims to be a reference for everyone who wants to introduce fixes to the existing code base. Most of the problems may be fixed easily using an automatic linter, others may need manual edits.

I would also like to add a status badge on the README like the example below:

But actually, even though we have lots of style violations the badge says 'passing' (at least currently). As that's not accurate yet, I won't add it yet.

Code snippet for this particular badge:

[![Java Style Checker](https://github.com/N0W0RK/TUM_GAD_Tests_SS21/actions/workflows/checkstyle.yml/badge.svg)](https://github.com/N0W0RK/TUM_GAD_Tests_SS21/actions/workflows/checkstyle.yml)