Update SimpleAtomicLongUnitTest.java to use longs
Made the skeleton for PlayPingPongThread a bit easier to understand:
 - replaced SimpleSemaphore array with to single SimpleSemaphores named "mIPlaySema" und "mYouPlaySema"
 - moved class variables from bottom to top of class
 - reformatted "TODO" comment for empty method calls to they do not have to be replaced by students