tuidoku VS tdoku

By pencil mode I mean the ability to annotate the board like you would do on normal paper. [Here, 2nd screenshot](https://github.com/flyingpeakock/tuidoku#screenshots) you can see what I mean. If you need such feature I suggest you to use in fact [tuidoku](https://github.com/flyingpeakock/tuidoku). I'm not sure if I'll implement it in sku.

For a casual effort this solver is quite fast. On most datasets it's significantly faster than a C++ port of the Norvig solver and it's often in the same league as well-tuned DLX solvers (though still not in the major league).

Relative to DLX it's faster on the easiest puzzles and slower on hard ones, though its performance degrades significantly in two specific still-easy cases: (1) 17-clue puzzles since these punish solvers that don't propagate hidden singles, (2) 0-solution puzzles since the solver doesn't check for consistency during initialization (something the author acknowledges in a comment).

For context see these benchmarks from a modern laptop (https://github.com/t-dillon/tdoku/blob/master/benchmarks/res...) and compare lhl_sudoku to the other simple solvers tdev_basic_heuristic, zerodoku, fast_solv_9r2, kudoku, and norvig.

For details on the solvers and datasets used in these benchmarks see here (https://github.com/t-dillon/tdoku/tree/master/other) and here (https://github.com/t-dillon/tdoku/tree/master/benchmarks)

https://github.com/t-dillon/tdoku (click VIEW CODE and download data zip for sudoku puzzles)