Added removeConstraint method to IWorldTree
Fixed a NPE bug in WorldTree.getAllChildren
Added method ITile.setPiece
Implemented this method in WorldTreeFactory.Tile

WorldTreeFactory has some minor optimizations
    - Constraint constructors now use default size 0
    - Tile.artifacts uses default size 0

Updated Region.initRegion method to use Tile.setPiece

WorldTreeFactory.materializeConstraints now contains some
commented code that removes all program-generated constraints

This method has been left commented as its
increased space complexity is insignificant (~3K per 2000 Tiles)