Debugs for KingCheckFilter. Looks like problem is with AttackGraph.
Adding debug printlns again.

It seems that the Observer pattern is not working as intended.
No longer checking if observedBoard hasChanged.

Also removing debugging printlns and it looks like we have a lot of null
pointers again!
TODO clean-up.
NullPointerExceptions remedied but there are King protection priority is still failing.
So much debugs and adding modifications to logic of KingCheckFilter.

The (previous) problem is that when we are "trying" the moves of the King, the
AttackGraph remains concerned about the previous location of the King. Hence
if the King moves to endanger himself, the filter does not notice.
Adding nother exception to the GameArbiterTest checks: the Knight that moved.

Tests now passing! Yay!
Cleaning up debugs, all tests passing at last!