Sudoku Solver

This is a Sudoku solver patterned from Peter Norvig's solver, along with an interface implemented with Tkinter, following examples found at newcoder.io. Changes include converting the main UI to use the grid component and including a Solve button. The solve method converts the existing grid object into a string passed to the solver, which returns a solved string. The UI's solve method then converts the solved string back into a working grid object and populates the grid.