Bug in Y and Z directions in 3D diffusion

[BekkerRA]

I was playing around with diffusion in 3D and noticed that the particles weren't behaving as expected. On investigation, the wrapping in functions DiffusionY3D and DiffusionZ3D in Tools/Internal/PopulationGridPDEequations.java handle wrapping incorrectly. Using DiffusionY3D as an example, the wrapping currently occurs on the x-direction

else if(WrapY){
x=Wrap(y,yDim);
int ct=mn.Sample(prob);
grid.Add(centerI,-ct);
grid.Add(xyDimzDim+y*zDim+z,ct);
}

but based on the DiffusionY2D function it should be:

else if(WrapY){
y=Wrap(y,yDim);
int ct=mn.Sample(prob);
grid.Add(centerI,-ct);
grid.Add(xyDimzDim+y*zDim+z,ct);
}