Randomization and Vectorization

[boisgera]

Improve the current state of the documentation. Both subjects should probably be handled in (appropriate subsections of) the "Calculus" section. Explain Constant (or randomize ? Both?), randomize for functions.

Not sure that the use of vectorized sampling should be explained here though. The def of vectorized function yes, but the use in sampling not so sure. This is "simulation", not (random-variable) "calculus". So do we need a "simulation" subsection ? There is little to talk about but vectorized sampling.

OK, so, at this stage : don't introduce a new section, consider that "calculus" is everything and document all vectorization issues there. We'll see later if and how the subjects can be dissociated.