Add check for negative copyFrom in TetrisBoard.clearRows
Implement TetrisBoard.decreaseHeights
Implement check for height consistency in TetrisBoard.sanityCheck
Add check for width consistency in TetrisBoard.sanityCheck
Add check for maxHeight consistency in TetrisBoard.sanityCheck
Add check for ragged board in TetrisBoard.sanityCheck
Add check for consistency in length of heights and widths arrays in
TB.sanityCheck