Create source code QA guidelines

[danshearer]

This is originally from @scp93ch .

Should we have a set of sonarlint rules that must pass? (for new code?)

Should we have a checkstyle ruleset that must be applied? (for new code?)

Comment originally from @danshearer : Whatever the answer, it can only indeed be for new code. Even though we will be visiting every file in the system-modeller tree, and even though we will be making at least one mass change, no, we will not be taking the opportunity to reformat code or correct spelling en masse. That breaks the ability to do diffs with old code, which causes great pain and possibly pestilence.