Javadoc errors

[rtaupe]

Generating javadoc using gradle javadoc yields many warnings and even errors, for example:

...\alpha-asp\Alpha\src\main\java\at\ac\tuwien\kr\alpha\grounder\Unifier.java:83: error: bad use of '>'
         * if left has A -> B and right has A -> t then the result will have A -> t and B -> t.