add how to run the solver to the readme

break: return grid directly instead of string, nicer printing.

refactor: change to standard file structure

Merge pull request #3 from LiorSinai/refactor/cleaniness

refactor: get row and col inds, apply_strategies

refactor: move code to separate get_least_candidates functions

refactor: change test names to standard format.

break: raise exception if there is an error on the board

feature: Hyper Sudoku, aka Windoku Sudoku

fix: more generic handling for special Sudokus like X Sudoku.

refactor: move functions to their own file

refactor: typing and formatting

refactor: separate tests

rename files

X-sudoko, error messages and camel case to snake case