Attach custom LGDO attributs via config file

[gipert]

At the moment there is no mechanism to attach user attributes to the output LGDOs via config file. The only supported way is through the decoded_values dictionary. Actually, I had implemented it but only for post-processed data (proc_spec section of the config). It would be nice to support this also for non-post-processed data (to e.g. set custom HDF5 settings for specific objects).