adds a series of blocks - normally more straight than
        random sea/land features - blocks are default 5x2

def add_blocks(self, perc_blocked=30):
        """  
        adds a series of blocks - normally more straight than
        random sea/land features - blocks are default 5x2
        """
        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()