This adds a new vertical viscosity parameterization as in Greatbatch and Lamb (1990),
Ferreira & Marshall (2006) and Zhao & Vallis (2008), hereafter referred to as the GL90
vertical viscosity parameterization. This vertical viscosity scheme redistributes momentum
in the vertical, and is the equivalent of the Gent & McWilliams (1990) parameterization,
but in a TWA (thickness-weighted averaged) set of equations. The vertical viscosity coefficient
nu is computed from kappa_GM via thermal wind balance, and the following relation:

nu = kappa_GM * f^2 / N^2.

The vertical viscosity del_z ( nu del_z u) is applied to the momentum equation with stress-free
boundary conditions at the top and bottom.

In the current implementation, kappa_GM is assumed either (a) constant or as (b) having an
EBT structure. A third possible formulation of nu is depth-independent:

nu = f^2 * alpha

The latter formulation would be equivalent to a kappa_GM that varies as N^2 with depth.
Currently, the GL90 parameterization is only implemented in stacked shallow water (SSW) mode,
in which case we have 1/N^2 = h/g'.

More specifically, this commit adds a new subroutine that computes the couping coefficient
associated with GL90 via

a_cpl_gl90 = nu / h = kappa_GM * f^2 / g'

or

a_cpl_gl90 = nu / h = f^2 * alpha / h.

Further, a_cpl_gl90 is multiplied by a function (botfn), which is 0 within the GL90 bottom boundary
layer, whose depth is set by Hbbl_gl90, and 1 otherwise. This modification is necessary to avlid fluxing
momentum into vanished layers that ride over steep topography.

Finally, a_cpl_gl90 is added to a_cpl, where the latter is the coupling coefficient associated with the
remaining vertical stresses, used in the vertical viscosity solver.

More information can be found in Loose et al. (https://www.essoar.org/doi/abs/10.1002/essoar.10512867.1),
Appendix B.

See PR mom-ocean#263

Change g' --> g^prime because doxygen and style test does not like
the apostrophe.

This variable is analogous to KHTH_USE_EBT_STRUCT, but is specifically for the GL90 scheme.
If the user sets KD_GL90_USE_EBT_STRUCT = True, an EBT structure will be applied to KD_GL90.
Before this commit, KHTH_USE_EBT_STRUCT took the two-fold role of imposing an EBT structure
on KHTH (used in the GM scheme) and KD_GL90 (used in the GL scheme), which may have been confusing.

All get_param calls that are for variables that are only used when
GL90 is in use get the argument do_not_log = .not.CS%use_GL90_in_SSW.
Prior, these variables have been wrapped by an if-statement.