PairCopulaKendallTau
Transforming the parameter of a pair-copula into the value of Kendall's tau
Purpose
The function computes the value of Kendall's tau given the parameter of the specific two-dimensional copula. Possible copula families:
       1   AMH
       7   Clayton
       8   FGM
       9   Frank
       10  Gaussian
       11  Gumbel
       19  t
Usage
           tau = CopulaKendallTau(family,theta)
Inputs
   family      = The copula family.
   theta       = The parameter vector for the pair-copula.
Outputs
  tau          = The value of Kendall's tau.