Add new depth input field

[JessicaMeixner-NOAA]

Is your feature request related to a problem? Please describe.
CESM uses a value of "1/5 of the mixed layer depth" to calculate other parameters in the output. To allow these output variables to be usable to all users of WW3, a new input of depth that can be defined optionally by the user needs to be added.

Describe the solution you'd like
A new input field that can be used in Langmuir related variable calculations.

Describe alternatives you've considered
A constant value could be defined via a namelist value and then set for every variable but then allow the actual depth to be a spatially varying variable that can be updated via coupling scenarios with active oceans if uncoupled scenarios do not make sense.

Additional context
This work is needed to merge #641 back to the develop branch.
FYI @qingli411