Sudokus are square, Sets are better
For some reason I didn't think about the rules of Sudoku when starting to design the board. Of course Sudokus are square, what do we need a columns and rows variable then? Sudokus just have a size parameter that indicates how many elements are in each column and row. Problem solved.
Also we deal with Sets instead of Lists now, because I like them more.
Documentation, Format
getParts (unfinished) + Test

Also variables in Board are now private. Things that should be modified have getters setters anyway.