Fix coordinates

[anton-seaice]

In the MOM6 code, it says this in a comment about the initial conditions:

  ! Read input grid coordinates for temperature and salinity field
  ! in z-coordinate dataset. The file is REQUIRED to contain the
  ! following:
  !
  ! dimension variables:
  !          lon (degrees_E), lat (degrees_N), depth(meters)
  ! variables:
  !          ptemp(lon,lat,depth) : degC, potential temperature
  !          salt (lon,lat,depth) : ppt, salinity
  !
  ! The first record will be read if there are multiple time levels.
  ! The observation grid MUST tile the model grid. If the model grid extends
  ! to the North/South Pole past the limits of the input data, they are extrapolated using the average
  ! value at the northernmost/southernmost latitude.

It appears this is not enforced, as our current initial conditions use different dimension names:

ncdump -hs /g/data/vk83/configurations/inputs/access-om3/mom/initial_conditions/global.1deg/2020.10.22/ocean_temp_salt.res.nc
netcdf ocean_temp_salt.res {
dimensions:
        GRID_X_T = 360 ;
        GRID_Y_T = 300 ;
        ZT = 50 ;
        time = UNLIMITED ; // (1 currently)
variables:
        double GRID_X_T(GRID_X_T) ;
                GRID_X_T:long_name = "Nominal Longitude of T-cell center" ;
                GRID_X_T:units = "degree_east" ;
                GRID_X_T:modulo = 360. ;
                GRID_X_T:point_spacing = "even" ;
                GRID_X_T:axis = "X" ;
                GRID_X_T:_Storage = "contiguous" ;
                GRID_X_T:_Endianness = "little" ;
        double GRID_Y_T(GRID_Y_T) ;
                GRID_Y_T:long_name = "Nominal Latitude of T-cell center" ;
                GRID_Y_T:units = "degree_north" ;
                GRID_Y_T:point_spacing = "uneven" ;
                GRID_Y_T:axis = "Y" ;
                GRID_Y_T:_Storage = "contiguous" ;
                GRID_Y_T:_Endianness = "little" ;
        double ZT(ZT) ;
                ZT:long_name = "zt" ;
                ZT:units = "meters" ;
                ZT:positive = "down" ;
                ZT:point_spacing = "uneven" ;
                ZT:axis = "Z" ;
                ZT:_Storage = "contiguous" ;
                ZT:_Endianness = "little" ;

We should change to the MOM names, as these data are on the MOM grid after the regridding.

Similarly, the time coordinates have the wrong metadata:

        double time(time) ;
                time:long_name = "time" ;
                time:units = "days since 0001-01-01 00:00:00" ;
                time:cartesian_axis = "T" ;
                time:calendar_type = "GREGORIAN" ;
                time:calendar = "GREGORIAN" ;
                time:_Storage = "chunked" ;
                time:_ChunkSizes = 512 ;
                time:_Endianness = "little" ;

We need to update this to use months since and use a valid calendar (Gregorian starts in 1582).

These two changes are in https://github.com/COSIMA/ocean-regrid/blob/e3e5404807c05ec4060624dcc0773f419860c8f0/util.py#L121

Once we've done that, we should use a cf-compliance checker on the output also.