add CRS metadata when encoding to netCDF

[Dimitar Misev]

Encoding output to netCDF from petascope does not contain any geo metadata.

For example, encode test_mean_summer_airtemp 2D coverage to netCDF returns this from ncdump:

ubyte Gray(Lon, Lat) ;
    Gray:valid_min = 0UB ;
    Gray:valid_max = 255UB ;
    Gray:description = "" ;
    Gray:units = "10^0" ;

It should rather contain something like this:

char crs ;
  crs:grid_mapping_name = "latitude_longitude" ;
  crs:long_name = "CRS definition" ;
  crs:longitude_of_prime_meridian = 0. ;
  crs:semi_major_axis = 6378137. ;
  crs:inverse_flattening = 298.257223563 ;
  crs:crs_wkt = "GEOGCS[\"WGS 84\",DATUM[\"WGS_1984\",SPHEROID[\"WGS 84\",6378137,298.257223563,AUTHORITY[\"EPSG\",\"7030\"]],AUTHORITY[\"EPSG\",\"6326\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.0174532925199433,AUTHORITY[\"EPSG\",\"9122\"]],AXIS[\"Latitude\",NORTH],AXIS[\"Longitude\",EAST],AUTHORITY[\"EPSG\",\"4326\"]]" ;
ubyte Gray(Lon, Lat) ;
  Gray:valid_min = 0UB ;
  Gray:valid_max = 255UB ;
  Gray:grid_mapping = "crs" ;
  Gray:description = "" ;
  Gray:units = "10^0" ;