Add to IODA example utility; write out a file

[CoryMartin-NOAA]

Adding to the IODA example, this PR now includes the mean value written out to a IODA file.

[CoryMartin-NOAA]

netcdf gmi_gpm_mean {
dimensions:
	Location = UNLIMITED ; // (1 currently)
variables:
	int Location(Location) ;
		Location:suggested_chunk_dim = 1LL ;

// global attributes:
		string :_ioda_layout = "ObsGroup" ;
		:_ioda_layout_version = 0 ;
data:

 Location = 0 ;

group: mean {

  group: ObsValue {
    variables:
    	float brightnessTemperature(Location) ;
    		brightnessTemperature:_FillValue = -999.f ;
    data:

     brightnessTemperature = 204 ;
    } // group ObsValue
  } // group mean
}

[RussTreadon-NOAA]

Useful example. Thanks for adding it to utils. Approve.

[guillaumevernieres]

Merci @CoryMartin-NOAA !