Cartesian product of constraint sets

[ManifoldFR]

The idea would be to introduce a child class of ConstraintSetBase<Scalar> which represents the Cartesian product of multiple constraint sets

$$\mathcal{C} = \mathcal{C}_1 \times \cdots\times \mathcal{C}_n$$

and implements the relevant projection and subdifferential operators.

This would be meant mainly for internal use in the solver and help simplify its implementation (eliminate loops, manual extraction of relevant segments/blocks in vectors/matrices).

[ManifoldFR]

I ended up adding this to aligator itself in the next release. I'll backport.