`Sequence.generate_random_poisson` does not generate IOIs

[YannickJadoul]

Sequence.generate_random_poisson generates a random sample of the number of events and uses it as IOIs. This does not seem to make sense, as it does not produce the time until the next sample, but just samples integers. This includes 0, which results in an error, cause 0 is not a valid IOI length.