Make random seed decorelated by default

[pyrollo]

Now, if no seed is specified in parameters, same random sequence is used.

This will lead to generation correlation. For example, using two random patterns with the same chance will place stuff exactly in the same place.

Such a behavior could be wanted but, in large majority of cases, an uncorrelated random sequence is desired. So we end up with a parameter file full of seed: specs.

It would be better to make seeds uncorrelated by default, with the possibility to correlate them by using the same seed: parameter.

This is not so simple because we have to choose seed wisely. Possible ways:

• Use Yaml path (not yet available from Jackson)
• Inherit seed form parent + salt (could be quite complex but allows to reproduce complex systems of nested seeds)
• Something else ?