Calculate steady-state Richardson number from c3

INTERFACE:

   REALTYPE function  compute_rist(c1,c2,c3)

DESCRIPTION:

Numerically computes the steady-state Richardson-number $Ri_{st}$ for two-equations models from the given $c_{\psi 3}$ and the parameters $c_{\psi 1}$ and $c_{\psi 2}$ according to (114). A (very tricky) double Newton-iteration is used to solve the resulting implicit non-linear equation.

USES:

   use turbulence, only:           as,an,cmue1,cmue2
   use turbulence, only:           cm0
   use turbulence, only:           turb_method,stab_method
   use turbulence, only:           cm0_fix,Prandtl0_fix
   use turbulence, only:           Constant
   use turbulence, only:           MunkAnderson
   use turbulence, only:           SchumGerz
   use turbulence, only:           EiflerSchrimpf
   IMPLICIT NONE

INPUT PARAMETERS:

   REALTYPE, intent(in)           :: c1,c2,c3

REVISION HISTORY:

   Original author(s): Hans Burchard, Lars Umlauf