Generates random integer values taken from a poisson distribution.
operator() method returns the next random value in the sequence. It takes one
parameter, the mean of the distribution. The mean should be positive for this operation
to make sense, otherwise the function will simply return 0.
The constructor takes and stores a reference to a
dip::Random object, which is used
to produce the randomness. That object needs to exist for as long as this one exists.