Develop and apply some code conventions (including null handling)

[Cervator]

Now that we've got Gestalt merged it is time to do some more cleanup.

Our usual code analytics config files are in place and I figure we'll follow the Terasology guidelines overall. Thanks to DestSol's smaller size we might be able to turn up the heat on the analytics detail and start using other sites and services that are more effort intensive to deal with in Terasology land.

• Decrease warning tolerances, maybe especially for Javadoc (Terasology's style is still pretty lenient since the slightest increase in warnings swamps the stats somewhat)
• Consider null handling like #130 and related discussion tried to (mostly on Slack?) - should we mandate that in a code convention after we sort out the details? It already has an entry in the PR template so we should do something with it
• Consider https://www.codefactor.io and related sites
• Consider newer options available via Jenkins, especially the more flexible commit status tweaking available via Jenkins pipeline / Jenkinsfile

[NicholasBatesNZ]

Note this scary plea in the wiki:

PLEASE DON'T USE THESE. Currently, this project aims to reduce amount of these as much as possible, by proper null policies.