Update Sudoku project and add two chapters

[ntBre]

I've been interested in making a Sudoku program for a while, so when I found out about Piston and saw there was already a tutorial on it, I was very excited. I noticed that I had to make a few changes to the code to get it to compile, so I went ahead and tried to update all of the code in the existing chapters. Then I was inspired to keep going with the project, so I added two more chapters covering

• loading an SDM file as a puzzle
• differentiating between loaded and input digits
• validating input
• detecting a solution

This is one of my first times contributing to a project, so apologies in advance for any issues or mistakes. And thanks for making Piston!

[bvssvni]

Merging

[bvssvni]

Thank you!