Fixed Rocket.pde: ArrayIndexOutOfBoundsException

Actually
`y*(width/gridscale)`should be the same as
`y*width/gridscale`.

But as `y` is of type `Integer`, it behaves differently and causes an `ArrayIndexOutOfBoundsException`.
Merge pull request #22 from mattii/master

Fixed Rocket.pde of EvolveFlowField: ArrayIndexOutOfBoundsException