code: def add_blocks(self, perc_blocked=30):
          
          self.refresh_stats()
          #print(self.show_grid_stats())
          while (100*(self.tot_blocked-10))/self.tot_pix < perc_blocked - 1:
              self.add_block()
              self.refresh_stats()
docs: adds a series of blocks - normally more straight than
          random sea/land features - blocks are default 5x2