Use bathymetry files or coupler input to determine the land mask

[eclare108213]

CICE reads POP kmt files to determine the land mask. There is also a bathymetry file available, which was created using bathymetry data from NEMO and is currently only used for the basal stress parameterization.

Some ocean models (e.g. MOM6 and NEMO6) have the capability to change the land mask during runtime; for this reason it would be useful to allow CICE's land mask information to arrive via the coupler.

While keeping kmt for backward capability, generalize determination of the land mask to include using bathymetry files and coupling input. Include namelist flags such as the filename, name of input variable on the file, and other control flags such as MASKING_DEPTH and MINIMUM_DEPTH (as used for MOM).

[anton-seaice]

I've been meaning to work on this for a while - I agree this would be a useful feature.

It is complicated slightly by ice-shelf cavities. Well want to support an extra mask field for ocean with an ice shelf over it.

MOM6 supports run time modifications of the landmask as well, we may not want to replicate all the MOM options:

TOPO_FILE = "topog.nc"          ! default = "topog.nc"
                                ! The file from which the bathymetry is read.
TOPO_VARNAME = "depth"          ! default = "depth"
                                ! The name of the bathymetry variable in TOPO_FILE.
TOPO_EDITS_FILE = ""            ! default = ""
                                ! The file from which to read a list of i,j,z topography overrides.
ALLOW_LANDMASK_CHANGES = False  !   [Boolean] default = False
                                ! If true, allow topography overrides to change land mask.
MINIMUM_DEPTH = 0.0             !   [m] default = 0.0
                                ! If MASKING_DEPTH is unspecified, then anything shallower than MINIMUM_DEPTH is
                                ! assumed to be land and all fluxes are masked out. If MASKING_DEPTH is
                                ! specified, then all depths shallower than MINIMUM_DEPTH but deeper than
                                ! MASKING_DEPTH are rounded to MINIMUM_DEPTH.
MASKING_DEPTH = -9999.0         !   [m] default = -9999.0
                                ! The depth below which to mask points as land points, for which all fluxes are
                                ! zeroed out. MASKING_DEPTH is ignored if it has the special default value.