Grid knows if all cells are solved
Cell has nil value when initialised
Cell knows if it has a unsolved value (0)
Cell knows if it has a solved value (not 0)
Cell knows if it's filled out or not
Grid knows who the neighbours of a cell are
Grid knows who a cell's neighbours are
Tested further cell values to identify row neighbours
Refactored row_neighbours method
Merged Toby-Sarah branch and fixed conflicts
Grid returns and empty board with 9 * 9 cells
Grid converts cells to nine by nine grid
Grid knows who a cell's row neighbours are
Grid returns column neighbours for a cell
Refactored Grid class
Grid organises cells into 3*3 boxes
Grid can list all neighbours for a particular cell
Cell provides solved value or options
Cell assigns solved cell to @value
Cell can solve itself