Return Gamma distributed random deviates according to:
            
                                  a-1  -bx
                            b (bx)    e
              p   (x) dx = ---------------- dx   for x > 0
               a,b             Gamma(a)
            
                         =  0                    otherwise
                                         //
             The arguments must satisfy the conditions:
             a > 0   (positive)
             b != 0  (non-zero)
            
             References:
            
             For parameter a >= 1 corresponds to algorithm GD in:
             J. H. Ahrens and U. Dieter, Generating Gamma Variates by a
             Modified Rejection Technique, Comm. ACM, 25, 1, 47-54 (1982).
             For parameter 0 < a < 1 corresponds to algorithm GS in:
             J. H. Ahrens and U. Dieter, Computer Methods for Sampling
             from Gamma, Beta, Poisson and Binomial Distributions,
             Computing, 12, 223-246 (1974).