Default constructors for simplicity

[TheCarpetMerchant]

Please provide a setter function on cells to easily set values, such as :

//...
cell.set('value');
cell.set(10);
//...

The setter using type checks to instantiate the correct CellValues. This will also help give the correct "cell style", because if I'm honest I can't be bothered to set a cell style for every int cell. The cells should at the very least have a default style if you want this system.
You made a type-driven system, which is great, but that means you type system must make it impossible to create values which inevitably throw Exceptions (such as cell.value = IntCellValue(10);).