Tidy up Java project

[adriangonz]

Context

The Java code on the engine currently has a mix of spaces and tabs, unused dependencies, deprecation notices, different coding styles, etc. It would be good to add linters which check for these (i.e. static checks) and auto-formatters which force the code to comply with a certain style guideline.

Suggestions

We can look into large Java projects (e.g. spring-framework) to see which tools they use. Ideally both the linters and autoformatters should be integrated with mvn and the former should get triggered by git hooks, to ensure that the conventions are always followed.

Ideally, the build should stop if any of the checks fails.