Count solutions

[jwg4]

This needs to be implemented in exact_cover first.

[Motiver7]

Hey ! I discovered your package because I was searching for a function that would count the solutions. Your package is exactly what I needed, so first of all thanks a lot for having done that work.

About this function, as you added it in exact_cover, you just need to add this piece of code in the TilingProblem class of the problem.py file:

    def count_solutions(self):
        self.make_problem()
        try:
            cnt = get_solution_count(self.array)
        except NoSolution:
            return None
        return cnt

That would also need a small change in your pyproject.toml: you required exact_cover == 1.3, but we need 1.5 for this.
I did not set up a PR just for that little piece of code, as I guess it would be more complicated to read than if I did this comment.

[jwg4]

Hi thanks very much for your kind remarks.

I would be very happy to accept a PR for this if you submit one. If not, I will add it soon.

Note that there are some exact cover packages which are more efficient than mine for counting solutions. I am also planning to incorporate their changes or to make polyomino work with them at some point.