code: def c_M_z(self, M, z):
          
          # fitted parameter values
          A = 5.22
          B = -0.072
          C = -0.42
          M_pivot = 2.*10**12
          return A*(M/M_pivot)**B*(1+z)**C
docs: fitting function of http://moriond.in2p3.fr/J08/proceedings/duffy.pdf for the mass and redshift dependence of the concentration parameter
  
          :param M: halo mass in M_sun/h
          :type M: float or numpy array
          :param z: redshift
          :type z: float >0
          :return: concentration parameter as float