implement delayed start for sims to mitigate file race conditions

[achubaty]

Rather than having to set this up at the project level, it'd be nice to specify a delayed start (e.g., using Sys.sleep()) when running simulations.

This is useful when starting multiple concurrent simulations which all access the same raster layers or other files.

[achubaty]

what makes the most sense for implementation? both simInit() and spades() could make use of this, but I don't want to hit a "double delay" when user runs both back-to-back or via simInitAndSpades().