Update Simulator.java

I remade gameGrid as HashMap for easier neighbor finding... you find the neighbor zone by x,y coords, get zone from HashMap by string key x +","+ Y, like:  "0,0" is top left grid square.

In general, I think gameGrid should be a class with its own methods of getNeighbors(), getNumDeer()etc.  Simulator should only runSim and maybe handle user interaction.