Update script which generates DES validation data to output coadd_object_ids

[urebbapr]

#44 made a change to label files for validation such that we should use a named id rather then the integer index of the example in the original data structure.

I need to change the script that lives in the des repo: src/find_discarded_objectids_in_dec2019_data_dora.py to output coadd_object_ids.py. Come to think of it, the original script src/find_discarded_objectids_in_dec2019_data.py might already do this.