227 false gets removed in transformations

[IgnaceBleukx]

Wasn't really sure about how to do it in pysdd...

[IgnaceBleukx]

After discussion offline: will implement a BoolVal wrapper so False can become a CPMpy expression. This will be part of a new transformation which also unravels all nested lists. Then every transformation will simply get a flat list of constraints and all transformations should work as is with the new BoolVal expression.
Will still have to add the cas of posting False to the solver in the _post_constraint of all solvers.

[tias]

We have a bunch of research code that expects flatten_constraint() to continue working also on a single expression.

I hence propose to make flatten_constraint() a semi-meta function that calls make_expr internally. That means solvers dont need to add the transform themselvs. I'll play with this on the plane.

[tias]

I reworked it some more. I think its looking nice, would be good to see this land rather soon, so we can base other branches on this.

It tested good on bigtest (ortools) too (no noticable timing differences though, a bit to my surprise)

@Wout4 and @IgnaceBleukx can you have a final look and merge?

[IgnaceBleukx]

Here check for merge_and, will add

[IgnaceBleukx]

Can confirm all tests run, for pysdd as well.
Ready to merge for me. @Wout4 waiting for your final thoughts before merging.

[Wout4]

looks good to me!