Add a getBlock() method to Batches

[MrGazdag]

This PR adds the getBlock(x, y, z) methods from Block.Getter to Batches, thus letting one query changed blocks from the block map. This is useful if you would like to pre-process batches before applying them to an instance. According to the default behavior of the internal maps, the methods return null if the position is not changed in the Batch.