Add JDoc comments

[norling]

Description

The Java code should have JDoc comments according to the google styleguide rules at https://google.github.io/styleguide/javaguide.html#s7-javadoc.

Proposed solution

Write JDoc where missing, and keep it updated for all future changes to the code.

Definition of Done

Javadoc is present for every public class, and every public or protected member of such a class.

[juhtornr]

Martin will make a PR when DataEdge part is done and then someone else can take a look and provide feedback.