Add Stokes forces

[qingli411]

1. Add Stokes vortex force, Stokes Coriolis force and pressure head in momentum equations, Stokes advection in tracer equations, and Stokes production in TKE equation
2. Add two Stokes drift options for deep water, all constant in time: exponential profile defined by the surface Stokes drift and Stokes depth, Stokes drift profile based on Donelan et al., 1985 spectrum

New control parameters (initialization) in the namelist to turn on Stokes forces:

• stokes_force: .FALSE. by default, set to .TRUE. to turn on the Stokes forces
• stokes_drift_method: 1 for exponential profile and 2 for Donelan et al., 1985 spectrum
• u0_stk, v0_stk and d_stk: x- and y-component of surface Stokes drift (m/s) and exponential decay depth (m), required if stokes_drift_method=1
• wind_speed, wind_dir and wave_age: 10-meter wind speed (m/s), wind direction (degree counter-clockwise from x-direction) and wave age, required if stokes_drift_method=2

[xylar]

@qingli411, I updated the title of the pull request (PR). I hope that's okay.

[xylar]

Once @vanroekel has approved the PR, @qingli411, you can feel free to merge it yourself by hitting the "Merge pull request" button. My suggestion is to paste in the description of the PR from above (your points 1 and 2) as part of the merge comment. Please leave the rest of the merge message as it is, since it's very useful to have the PR number in the commit message.

[qingli411]

Thanks, @xylar ! Just add some more description of the additional control parameters for the Stokes forces.

[qingli411]

@vanroekel I haven't done a side-by-side comparison with the NCAR LES but was planning to do so. I will post a comparison figure and probably also the test case namelists later.

[qingli411]

@vanroekel I just pushed my new modifications to the repository. In this version I compute the Stokes drift at w-levels and save them in variables u_stk_zw and v_stk_zw. Could you please take a look?