DOEScenario: remove the attribute seed

The notion of seed is related to an algorithm generating pseudo-random numbers. Some DOE algorithms rely on such algorithms.

Then, if a DOEScenario uses 2 DOELibrary, 2 seeds will be used (1 per DOELibrary).

But the current DOEScenario has 1 seed as attribute instead of 2.

I propose to remove the attribute DOEScenario.seed and pass custom seeds to DOE algorithms as algorithm options.