Add momentum-advection term, default is off.

- add place-holder for momentum-advection term in OceanModel.jl, (default = empty)
- add momentum advection in "simple_toy_channel.jl" and update refvals.

Apply formatter.

Increase horizontal viscosity in simple double-gyre test

- increase horizontal viscosity from 5.e+3 to 15.e+3 ; this allows to run for
  more than 20.yrs (and give reasonable solution).
- also update BC and generate new refvals.

Clean-up simple-box test for new split-explicit RK-3 scheme

- run for 3.days and add refvals.

Merge branch 'master' into jmc/split_explicit_draft

Update argument list of fct "init_state_auxiliary!"

Fix vertical velocity calculation

Also bring back an old bug (double call to filters) to get back older results.

Move gravity inside "pkin" to make it the kinematic pressure

Also use new variable name "buoy" (for buoyancy) as integrands, that can
easily account for salinity in EOS.
Update output reference value (since pkin has changed).

Update Split-Explicit testing scripts

- Update "simple_box_2dt.jl", still without convective adjustment (and no IVDC)
- New test script "simple_box_ivd.jl" with  convective adjustment and IVDC,
  with reference StateCheck values for a 5.d run.

Fix previous changes to work on GPU

Also improve some variable names from previous changes