Give the PositionInGrid class an ability produce its northern neighbor
Implemented PositionInGrid.northwesternNeighbor()
Implemented PositionInGrid.westernNeighbor()
Implemented PositionInGrid.southwesternNeighbor()
Implemented PositionInGrid.souternNeighbor()
Implemented PositionInGrid.southeasternNeighbor()
Implemented PositionInGrid.easternNeighbor()
Implemented PositionInGrid.northeasternNeighbor()