Prescribe Cd #77

For simple analytical test cases, it is useful to prescribe the transfer coefficients cdd, cde, cdh.

In order to handle this, would it be possible to add a third option in addition to "kondo" and "fairall" which could be "constant_cd" with 3 variables: cdd, chd, ced?

this could be done either independently: airsea_fluxes could look like

    select case (method)
      case (1) ! Kondo
         call kondo(sst,airt,u10,v10,precip,evap,taux,tauy,qe,qh)
      case (2) ! Fairall et. all
         call fairall(sst,airt,u10,v10,precip,evap,taux,tauy,qe,qh)
      case default
         call constant_cd(sst,airt,u10,v10,precip,evap,taux,tauy,qe,qh)
   end select

with an additional subroutine constant_cd

or within kondo and fairall: This can be easily done for instance at the end of these routines:

   if (constant_cd) then
      cdd = ... input value ...
      chd = ...
      ced = ...
    end if

I am happy to propose a pull request for option 1 or 2