VanillaFluidSimulator: Fixed an invalid Y-coord query.

This was causing a spam of console messages, along with possible server crash, when liquids passed below the world: http://forum.mc-server.org/showthread.php?tid=1508&pid=15632#pid15632
Rewritten block entity loading.

Block entities are now loaded based on the blocktype at the coords they specify; before loading, their type ("id" NBT tag) is checked.
The chunk now expects that all block entities given to it via cChunk::SetAllData() have their valid blocktype; asserts if they don't.
Fixes #1354.
cChunk: Fixed the Coords param.