simulation.alphaDecay([decay])
If decay is specified, sets the alpha decay rate to the specified number in the range [0,1] and returns this simulation. If decay is not specified, returns the current alpha decay rate, which defaults to 0.0228… = 1 - pow(0.001, 1 / 300) where 0.001 is the default minimum alpha.
The alpha decay rate determines how quickly the current alpha interpolates towards the desired target alpha; since the default target alpha is zero, by default this controls how quickly