Added Block and id/data overloads to canPassThrough, centralTopLimit and isNaturalTerrainBlock.
Made everything that uses canPassThrough use data values.
Flagged all snow layers except single layers (data values 0 and 8) as "cannot be passed through".
Made /ascend and /descend use centralTopLimit.
Fixed centralTopLimit for trap doors and fence gates.
Fixed centralTopLimit for cake blocks with data values 6..15.