Use spherical harmonic for regridding May produce riggles.
Take an instance of geodat.nc.Variable, Deduce the lat-lon grid on a complete sphere, Return a regridded data on a spherical grid (nlat,nlon)
Return: a geodat.nc.Variable instance
TODO: grid.regrid now only handle 2D or 3D data, extend the function to handle rank-3+ data by flattening the extra dimension into one dimension