Anisotropic viscosity: CS%KH_aniso = 2 k_a

[NoraLoose]

There is an inconsistency between

• how anisotropic viscosity is implemented in MOM_hor_visc and
• what is described in the documentation.

The two would be consistent if CS%KH_aniso is interpreted as 2 k_a. For instance, substituting this line
https://github.com/NOAA-GFDL/MOM6/blob/b92d763ec91d57d3868162d5d9a24827d2e53674/src/parameterizations/lateral/MOM_hor_visc.F90#L947
by

Kh(i,j) = Kh(i,j) + 2 k_a * (1. - CS%n1n2_h(i,j)**2) 

would be consistent with the documentation and with Smith & McWilliams, 2003. We also need CS%KH_aniso = 2 k_a here, here, and here.

In summary, the code is not wrong, but the user should specify CS%KH_aniso = 2 k_a (rather than CS%KH_aniso = k_a, where the latter is maybe what one would expect after reading the docs).