Add another distribution to the composite distribution. This will add the distribution for the next 'distributionSize'
values, after any previously added.
For example, calling addDistribution(10, X) once will result in values 0 to 9 (inclusive) being modelled
by the specified distribution X. Calling addDistribution(10, Y) after that will result in values 10 to 19 (inclusive)
being modelled by distribution Y.
distributionSize - Number of values to model with the specified distribution