Return log-normal distributed random deviates
             with given mean and standard deviation stdev
             according to the density function:
                                                            2
                                 1                (ln x - m)
             p   (x) dx =  -------------- exp( - ------------ ) dx  for x > 0
              m,s          sqrt(2 pi x) s               2
                                                     2 s
            
                        = 0   otherwise
            
             where  m  and  s  are related to the arguments mean and stdev by:
            
                                    2
                                mean
             m = ln ( --------------------- )
                                 2      2
                      sqrt( stdev + mean  )
            
                                 2      2
                            stdev + mean
             s = sqrt( ln( -------------- ) )
                                    2
                                mean