(file-handler)+implemented write_to_file

First line is the data width, then the sand particles of each cell per
row.

main.c

@@ -117,6 +117,8 @@ int main(void)
 
   if (state.thread_alive)
     pthread_cancel(step_thread);
+
+  write_to_file(&state, "data.out");
   CloseWindow();
   return 0;
 }