mosaic simulate: allow small values of '-c' / '-p'

[tobiasmarschall]

This currently does not work due to limitations in the boost implementation of the negative binomial distribution. But we need this to test smaller window sizes.

[meiers]

Reminder to myself: This does not work because the NB distribution in boost requires integer numbers as dispersion parameter r, hence it cannot be <1 and loses resolution already in the range <5.
Interestingly this is only the case for sampling from an NB, not for calculating the probability density (basically boost has two different NB implementations).
Maybe I can find a proper NB implementation somewhere else.