Add cell update dropout

[mturnshek]

from distill "Growing Neural Cellular Automata"
https://distill.pub/2020/growing-ca/

"Typical cellular automata update all cells simultaneously. This imples the existence of a global clock, synchronizing all cells. Relying on global synchronization is not something one expects from a self-organizing system. We relax this requirment by assuming that each cell performs an update independently, waiting for a random time interval between updates."