Where:
- imol is an integer number
- chain_id is a string
- resno is an integer number
- ins_code is a string
- atom_name is a string
- alt_conf is a string
- attribute_name is a string
- attribute_value is a string
set a string attibute to the atom with the given specifier.
Attributes can be "atom-name", "alt-conf", "element" or "segid".