Replace _FillValue with NC_FillValue

[jayeshkrishna]

Newer versions of the NetCDF library only supports NC_FillValue instead of _FillValue to avoid conflicts with the C++ library headers. We also need to move (with appropriate config checks) to using NC_FillValue.

Also see Unidata/netcdf-c#2858 , https://github.com/Unidata/netcdf-c/blob/main/RELEASE_NOTES.md

[wkliao]

I think the safest way for users of NetCDF4 is to use string "_FillValue" directly.
That way Scorpio can be built with any NetCDF4 versions.