Added an __iter__ method to BlockGrid and .row and .col attributes to Block.
Moved BlockGrid width and height attributes to read-only properties and added a shape property.
Adding __str__ methods to Block and BlockGrid.
Adding _repr_html_ and show methods to Block.
Changing the Blocks.set_colors method to take individual colors values instead of a tuple, and updating the __setitem__ code in BlockGrid accomodate that.