aod_viirs_obs_2018041500_sf42.nc4 contains NaN fill values

[ctgh]

aod_viirs_obs_2018041500_sf42.nc4 contains NaN fill values. For example:

group: ObsValue {
  variables:
  	float aerosol_optical_depth_4(Location) ;
  		aerosol_optical_depth_4:_FillValue = NaNf ;
  } // group ObsValue

These currently cause warnings in ioda ctests.