I fixed the Zone constructor's issues and got rid of getBlockCountInZone().

I fixed the Zone issues that you mentioned in the constructor and still
kept the ZoneCornersInDifferentWorldsException in place.

Also, I removed the getBlockCountInZone() method because it was pointless;
it does the exact same thing as getVolume(), but "getVolume" is a much
more concise and straightforward name.

Signed-off-by: REALDrummer <conman.davey@gmail.com>