xarray adds fill value when outputting to NetCDF

[ellesmith88]

Description

xarray adds _FillValue when outputting to NetCDF. See this example for lat

Before working with xarray:

double lat(lat) ;
                lat:bounds = "lat_bnds" ;
                lat:units = "degrees_north" ;
                lat:axis = "Y" ;
                lat:long_name = "latitude" ;
                lat:standard_name = "latitude" ;

After working with xarray:

double lat(lat) ;
                lat:_FillValue = NaN ;
                lat:bounds = "lat_bnds" ;
                lat:units = "degrees_north" ;
                lat:axis = "Y" ;
                lat:long_name = "latitude" ;
                lat:standard_name = "latitude" ;

By setting ds.lat.encoding['_FillValue'] = None before outputting to NetCDF, the lat:_FillValue = NaN ; can be removed. This is happening with the NetCDF files we are generating with roocs. CF conventions say that coordinate variables cannot have missing values, so we don't want this added _FillValue in this case.