Read in Tripolar Grids

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MET needs the ability to handle tripolar grids. The NOAA EMC Real-Time Ocean Forecast System (RTOFS) uses a tripolar grid. Tripolar grids are 'a composite of two grids connected along a specified "join" latitude' and were originally defined by Murray 1996. More simply, there is a bipolar grid on one side (north or south) of the join latitude and a single pole grid on the other side. For RTOFS, this join latitude is 47°N. North of 47°N is an Arctic bipolar grid and south of 47°N is a Mercator projection (see attached figure). Tripiolar grids are designed to mitigate the convergence of longitudes at the North Pole.

References:

1. Environmental Modeling Center: "About Global RTOFS". https://polar.ncep.noaa.gov/global/about/
2. Geophysical Fluid Dynamics Laboratory: "A Guide to Grid Coupling in FMS".
   https://www.gfdl.noaa.gov/guide-to-grid-coupling-in-fms/
3. Murray, R. J., 1996: Explicit generation of orthogonal grids for ocean models. Journal of Computational
   Physics, 126, 251-273.

RTOFS Model Grid

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RTOFS data is in netCDF format with CF conventions, according to their website.

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Duplicate of #1345.
Tested

1. Download a file from ftp://ftpprd.ncep.noaa.gov/pub/data/nccf/com/rtofs/prod/rtofs.20201201/rtofs_glo_2ds_f000_1hrly_diag.nc and ftp://ftpprd.ncep.noaa.gov/pub/data/nccf/com/rtofs/prod/rtofs.20201201/rtofs_glo_2ds_f012_1hrly_prog.nc
2. Run the command:

• ./point2grid rtofs_glo_2ds_f000_1hrly_diag.nc G212 ice_coverage.nc -field 'name="ice_coverage"; level="(0,*,*)";' -v 4
• /point2grid rtofs_glo_2ds_f012_1hrly_prog.nc G212 sst.nc -field 'name="sst"; level="(0,*,*)";' -v 4