Minor changes and bug fixes

-Made it impossible for there to be an odd number of derps
-Modified new tileset and consolidated it with the old one
-Removed smiley faces from plants as humanizing them made derps hesitant
to eat them on moral grounds
-Program will now shut down ungracefully if there are too many derps to
be contained in the world
-Changed output dialog to display NaN when there have not been enough
generations to display the long-running average