draw box & CellCube
bugfix in CellCube & lamda computation in PBF
enforce incompressiblity
collistion detection, imcompressity interation times, some minor
optimazatio
collisiton: with bounce back velocity
imcompressity: iterate three times. lamda e constant is crucial, the
bigger it is, the more stable it seems
optimization: replace r*r with lengthSquare.