A real value of type mpfr1 which approximates the expected number of
components
In spite of the high precision arithmetic packages used for in function, it
can be numerically unstable for small values of Gama. This is because
evaluating a sum with alternated signs, in the generalized factorial
coefficients, is tricky. Reference: P. De Blasi, S. Favaro, A. Lijoi, R. H.
Mena, I. Prünster, and M. Ruggiero, “Are gibbs-type priors the most natural
generalization of the Dirichlet process?,” IEEE Trans. Pattern Anal. Mach.
Intell., vol. 37, no. 2, pp. 212–229, 2015.