The EGM2008.nc grid was created via the interp_2p5min.f program from NGA.  I created
input the coordinates for a global 1x1 arc minute gridline registered grid.  I know the
resolution of the EGM2008 is only 2.5 arc minutes but for remote files I wish
to stick with the regular resolutions of 1m, 2m, etc.  This grid will serve
as the original for the gmtserver-admin recipe earth_geoid.recipe.

Paul Wessel, Oct 15, 2021.

Dec 29, 2021: Had to do a grdedit -S -Rd on the file to ensure -180/180 longitude range
like all the other files.