docs: def evolve(self, years):
          """
          run the evolution of the planet to see how it looks 
          after 'years'
          """
          world_file = fldr + os.sep + self.name + '.txt'
          self.build_base()
          self.world.add_mountains()
          self.add_life()
          self.world.grd.save(world_file)
          
          print('TODO - run ' + str(years) + ' years')