tests and methods for telling if the game is solved
Added a lookup fuction in cell.rb to give me each mini square for a row / col
Cell knows if it is solved or not
Refactor of Grid. Moved test for cell solved to cell
realised using references to cells in cells is not a good idea
implimented passing @cell_array to each object
added methods to get the values of all the cells in row/col/box of a given cell
Added remove zeros message
generates candidates, knows if there is only one candidate
Working version it solves an easy puzzle
refactor and tidy up