coordinate_mapping too complex for netcdf global atributes

[pgancarski]

The structure of coordinate_mapping can not be directly copied to the netcdf global attributes.
netcdf needs a flat structure var_name: value

metadata-schema/dataset_schema.yaml

Line 287 in 11bf581

coordinate_mapping:

[nikokaoja]

@pgancarski Can you come up with something that will work with NetCDF ?

[pgancarski]

coordinate_mapping_x
coordinate_mapping_y
coordinate_mapping_z

it will be consistent with other attributes names that we use
I will make the patch