Update to handling of missing values, both in the stats and in the netcdf writer.

update to netcdf writer to explicity declare _FillValue encoding for variables and error message for UserTiffData class when projections are not correctly specified.

Fixed all stats methods to use missing value (netcdf default_fill_val), when polygon is out of bounds, or partially filled and un-masked stat returns nan.

Updated stat's tests to reflect this change.

tested with conus gfv11 also